Intermolecular dissociation energies of 1-naphthol complexes with large dispersion-energy donors: Decalins and adamantane.

The ground-state intermolecular dissociation energies D0(S0) of supersonic-jet cooled intermolecular complexes of 1-naphthol (1NpOH) with the bi- and tricycloalkanes trans-decalin, cis-decalin, and adamantane were measured using the stimulated-emission-pumping/resonant two-photon ionization (SEP-R2PI) method. Using UV/UV holeburning, we identified two isomers (A and B) of the adamantane and trans-decalin complexes and four isomers (A-D) of the cis-decalin complex. For 1NpOH·adamantane A and B, the D0(S0) values are 21.6 ± 0.15 kJ/mol and 21.2 ± 0.32 kJ/mol, those of 1NpOH·trans-decalin A and B are 28.7 ± 0.3 kJ/mol and 28.1 ± 0.9 kJ/mol, and those of 1NpOH·cis-decalin A and B are 28.9 ± 0.15 kJ/mol and 28.7 ± 0.3 kJ/mol. Upon S0 → S1 electronic excitation of the 1NpOH moiety, the dissociation energies of adamantane, trans-decalin, and the cis-decalin isomer C change by <1% and those of cis-decalin isomers A, B, and D increase only slightly (1%-3%). This implies that the hydrocarbons are dispersively adsorbed to a naphthalene "face." Calculations using the dispersion-corrected density functional theory methods B97-D3 and B3LYP-D3 indeed predict that the stable structures have face geometries. The B97-D3 calculated D0(S0) values are within 1 kJ/mol of the experiment, while B3LYP-D3 predicts D0 values that are 1.4-3.3 kJ/mol larger. Although adamantane has been recommended as a "dispersion-energy donor," the binding energies of the trans- and cis-decalin adducts to 1NpOH are 30% larger than that of adamantane. In fact, the D0 value of 1NpOH·adamantane is close to that of 1NpOH·cyclohexane, reflecting the nearly identical contact layer between the two molecules.

Transition from Water Wires to Bifurcated H-Bond Networks in 2-Pyridone·(H2O) n, n = 1-4 Clusters.

Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O) n clusters with n = 1-4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm-1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1-4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

Do Hydrogen Bonds Influence Excitonic Splittings?

The excitonic splitting and vibronic quenching of the inversion-symmetric homodimers of benzonitrile, (BN)2, and meta-cyanophenol, (mCP)2, are investigated by two-color resonant two-photon ionization spectroscopy. These systems have very different hydrogen bond strengths: the OH···N≡C bonds in (mCP)2 are ∼10 times stronger than the CH···N≡C hydrogen bonds in (BN)2. In (BN)2 the S0((1)Ag) → S1((1)Ag) transition is electric-dipole forbidden, while the S0((1)Ag) → S2((1)Bu) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S0 → S1S2 spectra of the dimers are compared and their excitonic splittings and vibronic quenchings are investigated by measuring the (13)C-substituted heterodimer isotopomers, for which the centrosymmetry is broken and both transitions are allowed. The excitonic splittings are determined as Δexc = 2.1 cm(-1) for (BN)2 and Δexc = 7.3 cm(-1) for (mCP)2. The latter exhibits a much stronger vibronic quenching, as the purely electronic splitting resulting from ab initio calculations is determined to be Δcalc = 179 cm-1, while in (BN)2 the calculated splitting is Δcalc = 10 cm(-1). The monomer site-shifts upon dimerization and comparing certain vibrations that deform the hydrogen bonds confirm that the OH···N≡C hydrogen bond is much stronger than the CH···N≡C bond. We show that the H-bonds have large effects on the spectral shifts, but little or no influence on the excitonic splitting.

Intermolecular dissociation energies of dispersively bound 1-naphthol⋠cycloalkane complexes.

Intermolecular dissociation energies D0(S0) of the supersonic jet-cooled complexes of 1-naphthol (1NpOH) with cyclopentane, cyclohexane, and cycloheptane were determined to within <0.5% using the stimulated-emission pumping resonant two-photon ionization method. The ground state D0(S0) values are bracketed as 20.23±0.07 kJ/mol for 1NpOH⋅cyclopentane, 20.34±0.04 kJ/mol for 1NpOH⋅cyclohexane, and 22.07±0.10 kJ/mol for two isomers of 1NpOH⋅cycloheptane. Upon S0→S1 excitation of the 1-naphthol chromophore, the dissociation energies of the 1NpOH⋅cycloalkane complexes increase from 0.1% to 3%. Three dispersion-corrected density functional theory (DFT) methods predict that the cycloalkane moieties are dispersively bound to the naphthol face via London-type interactions, similar to the "face" isomer of the 1-naphthol⋅cyclopropane complex [S. Maity et al., J. Chem. Phys. 145, 164304 (2016)]. The experimental and calculated D0(S0) values of the cyclohexane and cyclopentane complexes are practically identical, although the polarizability of cyclohexane is ∼20% larger than that of cyclopentane. Investigation of the calculated pairwise atomic contributions to the D2 dispersion energy reveals that this is due to subtle details of the binding geometries of the cycloalkanes relative to the 1-naphthol ring. The B97-D3 DFT method predicts dissociation energies within about ±1% of experiment, including the cyclopropane face complex. The B3LYP-D3 and ωB97X-D calculated dissociation energies are 7-9 and 13-20% higher than the experimental D0(S0) values. Without dispersion correction, all the complexes are calculated to be unbound.

Experimental and Calculated Spectra of π-Stacked Mild Charge-Transfer Complexes: Jet-Cooled Perylene·(Tetrachloroethene)n, n = 1,2.

The S0 ↔ S1 spectra of the mild charge-transfer (CT) complexes perylene·tetrachloroethene (P·4ClE) and perylene·(tetrachloroethene)2 (P·(4ClE)2) are investigated by two-color resonant two-photon ionization (2C-R2PI) and dispersed fluorescence spectroscopy in supersonic jets. The S0 → S1 vibrationless transitions of P·4ClE and P·(4ClE)2 are shifted by δν = -451 and -858 cm(-1) relative to perylene, translating to excited-state dissociation energy increases of 5.4 and 10.3 kJ/mol, respectively. The red shift is ∼30% larger than that of perylene·trans-1,2-dichloroethene; therefore, the increase in chlorination increases the excited-state stabilization and CT character of the interaction, but the electronic excitation remains largely confined to the perylene moiety. The 2C-R2PI and fluorescence spectra of P·4ClE exhibit strong progressions in the perylene intramolecular twist (1au) vibration (42 cm(-1) in S0 and 55 cm(-1) in S1), signaling that perylene deforms along its twist coordinate upon electronic excitation. The intermolecular stretching (Tz) and internal rotation (Rc) vibrations are weak; therefore, the P·4ClE intermolecular potential energy surface (IPES) changes little during the S0 ↔ S1 transition. The minimum-energy structures and inter- and intramolecular vibrational frequencies of P·4ClE and P·(4ClE)2 are calculated with the dispersion-corrected density functional theory (DFT) methods B97-D3, ωB97X-D, M06, and M06-2X and the spin-consistent-scaled (SCS) variant of the approximate second-order coupled-cluster method, SCS-CC2. All methods predict the global minima to be π-stacked centered coplanar structures with the long axis of tetrachloroethene rotated by τ ≈ 60° relative to the perylene long axis. The calculated binding energies are in the range of -D0 = 28-35 kJ/mol. A second minimum is predicted with τ ≈ 25°, with ∼1 kJ/mol smaller binding energy. Although both monomers are achiral, both the P·4ClE and P·(4ClE)2 complexes are chiral. The best agreement for adiabatic excitation energies and vibrational frequencies is observed for the ωB97X-D and M06-2X DFT methods.

Modeling the Histidine-Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene.

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ∼80% of the T-shaped histidine···aromatic contacts are CH···π, and only ∼20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C6 axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by -73 cm(-1) relative to that of bare imidazole at 3518 cm(-1), indicating a moderately strong NH···π interaction. While the S0(A1g) → S1(B2u) origin of benzene at 38 086 cm(­1) is forbidden in the gas phase, Im·Bz exhibits a moderately intense S0 → S1 origin, which appears via the D(6h) → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm­1). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)2 cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz2 are cleanly separable by IR hole burning. The UV spectrum of Im·Bz2 exhibits two 000 bands corresponding to the S0 → S1 excitations of the two inequivalent benzenes, which are symmetrically shifted by -86/+88 cm(-1) relative to the 000 band of benzene

Analysis of the S2←S0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach.

The S2←S0 vibronic spectrum of the ortho-cyanophenol dimer (oCP)2 is analyzed in a joint experimental and theoretical investigation. Vibronic excitation energies up to 750 cm(-1) are covered, which extends our previous analysis of the quenching of the excitonic splitting in this and related species [Kopec et al., J. Chem. Phys. 137, 184312 (2012)]. As we demonstrate, this necessitates an extension of the coupling model. Accordingly, we compute the potential energy surfaces of the ortho-cyanophenol dimer (oCP)2 along all relevant normal modes using the approximate second-order coupled cluster method RI-CC2 and extract the corresponding coupling constants using the linear and quadratic vibronic coupling scheme. These serve as the basis to calculate the vibronic spectrum. The theoretical results are found to be in good agreement with the experimental highly resolved resonant two-photon ionization spectrum. This allows to interpret key features of the excitonic and vibronic interactions in terms of nodal patterns of the underlying vibronic wave functions.

Excitonic splittings in molecular dimers: why static ab initio calculations cannot match them.

After decades of research on molecular excitons, only few molecular dimers are available on which exciton and vibronic coupling theories can be rigorously tested. In centrosymmetric H-bonded dimers consisting of identical (hetero)aromatic chromophores, the monomer electronic transition dipole moment vectors subtract or add, yielding S0 → S1 and S0 → S2 transitions that are symmetry-forbidden or -allowed, respectively. Symmetry breaking by 12C/13C or H/D isotopic substitution renders the forbidden transition weakly allowed. The excitonic coupling (Davydov splitting) can then be measured between the S0 → S1 and S0 → S2 vibrationless bands. We discuss the mass-specific excitonic spectra of five H-bonded dimers that are supersonically cooled to a few K and investigated using two-color resonant two-photon ionization spectroscopy. The excitonic splittings Δcalc predicted by ab initio methods are 5-25 times larger than the experimental excitonic splittings Δexp. The purely electronic ab initio splittings need to be reduced ("quenched"), reflecting the coupling of the electronic transition to the optically active vibrations of the monomers. The so-called quenching factors Γ < 1 can be determined from experiment (Γexp) and/or calculation (Γcalc). The vibronically quenched splittings Γ·Δcalc are found to nicely reproduce the experimental exciton splittings.

Excitonic splitting, delocalization, and vibronic quenching in the benzonitrile dimer.

The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its (13)C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 → S2 electronic origin is observed, while the S0 → S1 excitonic component is electric-dipole forbidden. A single (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 ­ h5(13)C), (BN)2-(h5 ­ d5), and (BN)2-(h5 ­ h5(13)C) exhibit both S0 → S1 and S0 → S2 origins. Isotope-dependent contributions Δiso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from Δiso((12)C/(13)C) = 3.3 cm(­1) to Δiso(h5/d5) = 155.6 cm(­)1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting Δexc = 2.1 ± 0.1 cm(­1). Since Δiso(h5/d5) ≫ Δexc and Δiso((12)C/(13)C) > Δexc, complete and near-complete exciton localization occurs upon (12)C/(13)C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 → S1 and S0 → S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is Δel(calc) = 10 cm(­1). This electronic splitting is reduced by the vibronic quenching factor Γ. The vibronically quenched exciton splitting Δel(calc)·Γ = Δvibron(calc) = 2.13 cm(­1) is in excellent agreement with the observed splitting Δexc = 2.1 cm(­1). The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 ­ d5) heterodimer.

Watson-Crick and sugar-edge base pairing of cytosine in the gas phase: UV and infrared spectra of cytosine·2-pyridone.

While keto-amino cytosine is the dominant species in aqueous solution, spectroscopic studies in molecular beams and in noble gas matrices show that other cytosine tautomers prevail in apolar environments. Each of these offers two or three H-bonding sites (Watson-Crick, wobble, sugar-edge). The mass- and isomer-specific S1 ← S0 vibronic spectra of cytosine·2-pyridone (Cyt·2PY) and 1-methylcytosine·2PY are measured using UV laser resonant two-photon ionization (R2PI), UV/UV depletion, and IR depletion spectroscopy. The UV spectra of the Watson-Crick and sugar-edge isomers of Cyt·2PY are separated using UV/UV spectral hole-burning. Five different isomers of Cyt·2PY are observed in a supersonic beam. We show that the Watson-Crick and sugar-edge dimers of keto-amino cytosine with 2PY are the most abundant in the beam, although keto-amino-cytosine is only the third most abundant tautomer in the gas phase. We identify the different isomers by combining three different diagnostic tools: (1) methylation of the cytosine N1-H group prevents formation of both the sugar-edge and wobble isomers and gives the Watson-Crick isomer exclusively. (2) The calculated ground state binding and dissociation energies, relative gas-phase abundances, excitation and the ionization energies are in agreement with the assignment of the dominant Cyt·2PY isomers to the Watson-Crick and sugar-edge complexes of keto-amino cytosine. (3) The comparison of calculated ground state vibrational frequencies to the experimental IR spectra in the carbonyl stretch and NH/OH/CH stretch ranges strengthen this identification.

Structure and intermolecular vibrations of perylene·trans-1,2-dichloroethene, a weak charge-transfer complex.

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the "hindered-rotation" Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(­1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) "twisting" vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked "centered" structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(­1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck­Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

NH3 as a strong H-bond donor in singly- and doubly-bridged ammonia solvent clusters: 2-pyridone·(NH3)(n), n = 1-3.

Mass- and isomer-selected infrared spectra of 2-pyridone·(NH3)n clusters with n = 1-3 were measured in the NH and CH stretch fundamental region (2400-3700 cm(-1)) using infrared (IR) laser depletion spectroscopy combined with resonant two-photon ionization UV laser detection. The IR depletion spectra reveal three different H-bonding topologies of these clusters: The n = 1 and 2 clusters form ammonia bridges stretching from the N-H to the C═O group of the cis-amide function of 2-pyridone (2PY), giving rise to intense and strongly red-shifted (2PY)NH and ammonia NH stretch bands. For n = 3, two isomers (3X and 3Y) are observed in the IR spectra: The spectrum of 3X is compatible with an ammonia-bridge structure like n = 2, with the third NH3 accepting an H-bond from C(6)-H of 2PY. The IR spectrum of 3Y exhibits a broad IR band in the 2500-3000 cm(-1) range and is characteristic of a bifurcated double-bridged structure in which the first NH3 accepts an H-bond from the (2PY)NH and donates two H-bonds to the other two ammonias, both of which donate to the C═O group of 2PY. This double-donor/double-bridge H-bonding pattern increases the acceptor strength of the first ammonia and dramatically lowers the (2PY)NH stretching frequency to ∼2700 cm(-1). For all clusters the ammonia 2ν4 HNH bend overtones in the 3180-3320 cm(-1) region gain intensity by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia NH stretches, which are red-shifted into the 3250-3350 cm(-1) region. The experimental results are supported by optimized structures, vibrational frequencies, and IR intensities calculated using density-functional theory with the B3LYP and PW91 functionals, as well as with the more recent functionals B97-D and M06-2X, which are designed to include long-range dispersive interactions.

Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer.

The benzoic acid dimer, (BZA)(2), is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S(1)/S(2) state splitting and coherent electronic energy transfer within supersonically cooled (BZA)(2) and its (13)C-, d(1) -, d(2) -, and (13)C/d(1) - isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)(2)-(h - h) and (BZA)(2)-(d - d) dimers are C(2h) symmetric, hence only the S(2) ← S(0) transition can be observed, the S(1) ← S(0) transition being strictly electric-dipole forbidden. A single (12)C/(13)C or H/D isotopic substitution reduces the symmetry of the dimer to C(s), so that the isotopic heterodimers (BZA)(2) - (13)C, (BZA)(2) -(h - d), (BZA)(2) -(h(13)C-d), and (BZA)(2) -(h - d(13)C) show both S(1) ← S(0) and S(2) ← S(0) bands. The S(1)/S(2) exciton splitting inferred is Δ(exc) = 0.94 ± 0.1 cm(-1). This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δ(iso), arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δ(iso) = 3.3 cm(-1) upon (12)C/(13)C substitution to 14.8 cm(-1) for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S(1) ← S(0) and S(2) ← S(0) origin bands; near-complete localization is observed even for a single (12)C/(13)C substitution. The S(1)/ S(2) energy gap of (BZA)(2) is Δ(calc) (exc)=11 cm(-1) when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Δ(vibron) (exc)=2.1 cm(-1) [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Δ(exc) = 0.94 cm(-1). The observed excitonic splittings can be converted to exciton hopping times τ(exc). For the (BZA)(2)-(h - h) homodimer τ(exc) = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)(2) in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)(2)(∗).

Vibrational quenching of excitonic splittings in H-bonded molecular dimers: adiabatic description and effective mode approximation.

The quenching of the excitonic splitting in hydrogen-bonded molecular dimers has been explained recently in terms of exciton coupling theory, involving Förster's degenerate perturbation theoretical approach [P. Ottiger, S. Leutwyler, and H. Köppel, J. Chem. Phys. 136, 174308 (2012)]. Here we provide an alternative explanation based on the properties of the adiabatic potential energy surfaces. In the proper limit, the lower of these surfaces exhibits a double-minimum shape, with an asymmetric distortion that destroys the geometric equivalence of the excitonically coupled monomers. An effective mode is introduced that exactly reproduces the energy gain and amount of distortion that occurs in a multi-dimensional normal coordinate space. This allows to describe the quenched exciton splitting as the energy difference of the two (S(1) and S(2)) vibronic band origins in a one-dimensional (rather than multi-dimensional) vibronic calculation. The agreement with the earlier result (based on Förster theory) is excellent for all five relevant cases studied. A simple rationale for the quenched exciton splitting as nonadiabatic tunneling splitting on the lower double-minimum potential energy surface is given.

Vibrational quenching of excitonic splittings in H-bonded molecular dimers: the electronic Davydov splittings cannot match experiment.

The S(1)/S(2) state exciton splittings of symmetric doubly hydrogen-bonded gas-phase dimers provide spectroscopic benchmarks for the excited-state electronic couplings between UV chromophores. These have important implications for electronic energy transfer in multichromophoric systems ranging from photosynthetic light-harvesting antennae to photosynthetic reaction centers, conjugated polymers, molecular crystals, and nucleic acids. We provide laser spectroscopic data on the S(1)/S(2) excitonic splitting Δ(exp) of the doubly H-bonded o-cyanophenol (oCP) dimer and compare to the splittings of the dimers of (2-aminopyridine)(2), [(2AP)(2)], (2-pyridone)(2), [(2PY)(2)], (benzoic acid)(2), [(BZA)(2)], and (benzonitrile)(2), [(BN)(2)]. The experimental S(1)/S(2) excitonic splittings are Δ(exp) = 16.4 cm(-1) for (oCP)(2), 11.5 cm(-1) for (2AP)(2), 43.5 cm(-1) for (2PY)(2), and <1 cm(-1) for (BZA)(2). In contrast, the vertical S(1)/S(2) energy gaps Δ(calc) calculated by the approximate second-order coupled cluster (CC2) method for the same dimers are 10-40 times larger than the Δ(exp) values. The qualitative failure of this and other ab initio methods to reproduce the exciton splitting Δ(exp) arises from the Born-Oppenheimer (BO) approximation, which implicitly assumes the strong-coupling case and cannot be employed to evaluate excitonic splittings of systems that are in the weak-coupling limit. Given typical H-bond distances and oscillator strengths, the majority of H-bonded dimers lie in the weak-coupling limit. In this case, the monomer electronic-vibrational coupling upon electronic excitation must be accounted for; the excitonic splittings arise between the vibronic (and not the electronic) transitions. The discrepancy between the BO-based splittings Δ(calc) and the much smaller experimental Δ(exp) values is resolved by taking into account the quenching of the BO splitting by the intramolecular vibronic coupling in the monomer S(1) ← S(0) excitation. The vibrational quenching factors Γ for the five dimers (oCP)(2), (2AP)(2), (2AP)(2), (BN)(2), and (BZA)(2) lie in the range Γ = 0.03-0.2. The quenched excitonic splittings Γ[middle dot]Δ(calc) are found to be in very good agreement with the observed splittings Δ(exp). The vibrational quenching approach predicts reliable Δ(exp) values for the investigated dimers, confirms the importance of vibrational quenching of the electronic Davydov splittings, and provides a sound basis for predicting realistic exciton splittings in multichromophoric systems.

The S1/S2 exciton interaction in 2-pyridone·6-methyl-2-pyridone: Davydov splitting, vibronic coupling, and vibronic quenching.

The excitonic splitting between the S(1) and S(2) electronic states of the doubly hydrogen-bonded dimer 2-pyridone[middle dot]6-methyl-2-pyridone (2PY·6M2PY) is studied in a supersonic jet, applying two-color resonant two-photon ionization (2C-R2PI), UV-UV depletion, and dispersed fluorescence spectroscopies. In contrast to the C(2h) symmetric (2-pyridone)(2) homodimer, in which the S(1) ← S(0) transition is symmetry-forbidden but the S(2) ← S(0) transition is allowed, the symmetry-breaking by the additional methyl group in 2PY·6M2PY leads to the appearance of both the S(1) and S(2) origins, which are separated by Δ(exp) = 154 cm(-1). When combined with the separation of the S(1) ← S(0) excitations of 6M2PY and 2PY, which is δ = 102 cm(-1), one obtains an S(1)/S(2) exciton coupling matrix element of V(AB, el) = 57 cm(-1) in a Frenkel-Davydov exciton model. The vibronic couplings in the S(1)/S(2) ← S(0) spectrum of 2PY·6M2PY are treated by the Fulton-Gouterman single-mode model. We consider independent couplings to the intramolecular 6a(') vibration and to the intermolecular σ(') stretch, and obtain a semi-quantitative fit to the observed spectrum. The dimensionless excitonic couplings are C(6a(')) = 0.15 and C(σ(')) = 0.05, which places this dimer in the weak-coupling limit. However, the S(1)/S(2) state exciton splittings Δ(calc) calculated by the configuration interaction singles method (CIS), time-dependent Hartree-Fock (TD-HF), and approximate second-order coupled-cluster method (CC2) are between 1100 and 1450 cm(-1), or seven to nine times larger than observed. These huge errors result from the neglect of the coupling to the optically active intra- and intermolecular vibrations of the dimer, which lead to vibronic quenching of the purely electronic excitonic splitting. For 2PY·6M2PY the electronic splitting is quenched by a factor of ~30 (i.e., the vibronic quenching factor is Γ(exp) = 0.035), which brings the calculated splittings into close agreement with the experimentally observed value. The 2C-R2PI and fluorescence spectra of the tautomeric species 2-hydroxypyridine·6-methyl-2-pyridone (2HP·6M2PY) are also observed and assigned.

Excitonic splittings in jet-cooled molecular dimers.

In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced.

N-H···π hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole-benzene.

The N-H···π hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr·Bz, Pyr·Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H···π hydrogen bond to the benzene ring. The pyrrole is tipped by ω(S(0)) = ±13° relative to the surface normal of Bz. The N···ring distance is 3.13 Å. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle ω(S(1)) = ±21°. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H···π interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1)←S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr·Bz in the same region exhibits a weak 0 band that is red-shifted by 58 cm(-1) relative to that of Bz (38 086 cm(-1)). The origin appears due to symmetry-breaking of the π-electron system of Bz by the asymmetric pyrrole NH···π hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0 band. The Bz moiety in Pyr·Bz exhibits a 6a band at 0 + 518 cm(-1) that is about 20× more intense than the origin band. The symmetry breaking by the NH···π hydrogen bond splits the degeneracy of the ν(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by ∼6 cm(-1). Both the 0 and 6 bands of Pyr·Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration ω', in agreement with the change of the ω tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.

Intermolecular clamping by hydrogen bonds: 2-pyridone⋠NH3.

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH(3) has been performed. The S(1)←S(0) spectrum extends up to ≈1200 cm(-1) above the 0(0) (0) band, close to twice the range observed for 2PY. The S(1) state nonradiative decay for vibrations above ≈300 cm(-1) in the NH(3) complex is dramatically slowed down relative to bare 2PY. Also, the Δv=2, 4,… overtone bands of the ν(1) ' and ν(2) ' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY⋅NH3, which implies that the bridging (2PY)NH⋅⋅⋅NH(3) and H(2) NH⋅⋅⋅O=C H-bonds clamp the 2PY at a planar geometry in the S(1) state. The mass-resolved UV vibronic spectra of jet-cooled 2PY⋅NH(3) and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S(0) and S(1) equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S(1)←S(0) vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C(S) symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H(2) NH⋅⋅⋅O=C H-bond distance is ≈0.7 Å shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H(2) NH⋅⋅⋅O distance to increase by 0.2 Å upon S(1)←S(0) electronic excitation, while the (2PY)NH⋅⋅⋅NH(3) H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

Large-amplitude vibrations of an N-H...pi hydrogen bonded cis-amide-benzene complex.

The ground-state N-H...pi interaction of 2-pyridone.benzene (2PY.Bz) has been studied by infrared-UV depletion spectroscopy of the supersonic-jet cooled complex [P. Ottiger et al., J. Phys. Chem. B (2009) 113, 2937]. Here, we investigate the large-amplitude vibrations of 2PY.Bz and its d(1)-2PY and benzene-d(6) isotopologues in the S(1) state, using two-color resonant two-photon ionization and UV-holeburning spectroscopies, complemented by RI-CC2 and SCS-RI-CC2 calculations of the S(1) state. The latter predict a tilted T-shaped structure with an N-H...pi hydrogen bond to the benzene ring, similar to the S(0) state. The binding energy is predicted to increase by 1.5 kJ mol(-1) upon S(1)<--S(0) excitation, in close agreement with the experimental value of 1.2 kJ mol(-1). The vibronic band structure up to 60 cm(-1) above the 0 band is dominated by large-amplitude delta tilting excitations, reflecting a change in the tilt angle of the T-shaped complex. The S(0) and S(1) state delta potentials were fitted to experiment, yielding a single minimum in the S(0) state and a double-minimum S(1) potential with delta(min) = +/-13 degrees. The second large-amplitude vibration is the theta twisting or benzene internal-rotation mode. Due to the C(6) symmetry of the benzene moiety the S(0) and S(1) state theta potentials are sixfold symmetric. Analysis of the theta band structure reveals that the S(0) and S(1)theta potentials are mutually aligned and that the internal rotation barriers are V(6)(S(0)) < 0.2 kJ mol(-1) and V(6)(S(1)) = 0.10(1) kJ mol(-1), in close agreement with the calculations. Weaker excitations of the totally symmetric intermolecular vibrations chi (shear), omega (bend) and sigma (stretch) vibrations are also observed. The 2PY intramolecular nu(1) overtone, corresponding to an 2PY amide out-of-plane twist distortion, lies approximately 30% higher than in bare 2PY, reflecting the hindrance of this motion by the strong N-H...pi interaction.