Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone.

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone.

Avobenzone (AB) is a widely used UVA filter known to undergo irreversible photodegradation. Here, we investigate the detailed pathways by which AB photodegrades by applying UV laser-interfaced mass spectrometry to protonated AB ions. Gas-phase infrared multiple-photon dissociation (IRMPD) spectra obtained with the free electron laser for infrared experiments, FELIX, (600-1800 cm-1) are also presented to confirm the geometric structures. The UV gas-phase absorption spectrum (2.5-5 eV) of protonated AB contains bands that correspond to selective excitation of either the enol or diketo forms, allowing us to probe the resulting, tautomer-dependent photochemistry. Numerous photofragments (i.e., photodegradants) are directly identified for the first time, with m/z 135 and 161 dominating, and m/z 146 and 177 also appearing prominently. Analysis of the production spectra of these photofragments reveals that that strong enol to keto photoisomerism is occurring, and that protonation significantly disrupts the stability of the enol (UVA active) tautomer. Close comparison of fragment ion yields with the TD-DFT-calculated absorption spectra give detailed information on the location and identity of the dissociative excited state surfaces, and thus provide new insight into the photodegradation pathways of avobenzone, and photoisomerization of the wider class of ß-diketone containing molecules.

Using hyperpolarised NMR and DFT to rationalise the unexpected hydrogenation of quinazoline to 3,4-dihydroquinazoline.

PHIP and SABRE hyperpolarized NMR methods are used to follow the unexpected metal-catalysed hydrogenation of quinazoline (Qu) to 3,4-dihydroquinazoline as the sole product. A solution of [IrCl(IMes)(COD)] in dichloromethane reacts with H2 and Qu to form [IrCl(H)2(IMes)(Qu)2] (2). The addition of methanol then results in its conversion to [Ir(H)2(IMes)(Qu)3]Cl (3) which catalyses the hydrogenation reaction. Density functional theory calculations are used to rationalise a proposed outer sphere mechanism in which (3) converts to [IrCl(H)2(H2)(IMes)(Qu)2]Cl (4) and neutral [Ir(H)3(IMes)(Qu)2] (6), both of which are involved in the formation of 3,4-dihydroquinazoline via the stepwise transfer of H+ and H-, with H2 identified as the reductant. Successive ligand exchange in 3 results in the production of thermodynamically stable [Ir(H)2(IMes)(3,4-dihydroquinazoline)3]Cl (5).

Strategies for the hyperpolarization of acetonitrile and related ligands by SABRE.

We report on a strategy for using SABRE (signal amplification by reversible exchange) for polarizing (1)H and (13)C nuclei of weakly interacting ligands which possess biologically relevant and nonaromatic motifs. We first demonstrate this via the polarization of acetonitrile, using Ir(IMes)(COD)Cl as the catalyst precursor, and confirm that the route to hyperpolarization transfer is via the J-coupling network. We extend this work to the polarization of propionitrile, benzylnitrile, benzonitrile, and trans-3-hexenedinitrile in order to assess its generality. In the (1)H NMR spectrum, the signal for acetonitrile is enhanced 8-fold over its thermal counterpart when [Ir(H)2(IMes)(MeCN)3](+) is the catalyst. Upon addition of pyridine or pyridine-d5, the active catalyst changes to [Ir(H)2(IMes)(py)2(MeCN)](+) and the resulting acetonitrile (1)H signal enhancement increases to 20- and 60-fold, respectively. In (13)C NMR studies, polarization transfers optimally to the quaternary (13)C nucleus of MeCN while the methyl (13)C is hardly polarized. Transfer to (13)C is shown to occur first via the (1)H-(1)H coupling between the hydrides and the methyl protons and then via either the (2)J or (1)J couplings to the respective (13)Cs, of which the (2)J route is more efficient. These experimental results are rationalized through a theoretical treatment which shows excellent agreement with experiment. In the case of MeCN, longitudinal two-spin orders between pairs of (1)H nuclei in the three-spin methyl group are created. Two-spin order states, between the (1)H and (13)C nuclei, are also created, and their existence is confirmed for Me(13)CN in both the (1)H and (13)C NMR spectra using the Only Parahydrogen Spectroscopy protocol.

Detection of unusual reaction intermediates during the conversion of W(N2)2(dppe)2 to W(H)4(dppe)2 and of H2O into H2.

W(N(2))(2)(dppe-κ(2)P)(2) reacts with H(2) to form WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P) and then W(H)(4)(dppe-κ(2)P)(2). When para-hydrogen is used in this study, polarized hydride signals are seen for these two species. The reaction is complicated by the fact that trace amounts of water lead to the formation of H(2), PPh(2)CH(2)CH(2)Ph(2)P(O) and W(H)(3)(OH)(dppe-κ(2)P)(2), the latter of which reacts further via H(2)O elimination to form W(H)(4)(dppe-κ(2)P)(2) and [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)]. These studies demonstrate a role for the 14-electron intermediate W(dppe-κ(2)P)(2) in the CH activation reaction pathway leading to [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-k(2)P}(dppe-k(2)P)]. UV irradiation of W(H)(4)(dppe-κ(2)P)(2) under H(2) led to phosphine dechelation and the formation of W(H)(6)(dppe-k(2)P)(dppe-k(1)P) rather than H(2) loss and W(H)(2)(dppe-κ(2)P)(2) as expected. Parallel DFT studies using the simplified model system W(N(2))(2)((Ph)HPCH(2)CH(2)PH(2)-κ(2)P)(H(2)PCH(2)CH(2)PH(2)-κ(2)P) confirm that ortho-metalation is viable via both W(dppe-κ(2)P)(2) and W(H)(2)(dppe-κ(2)P)(2) with explicit THF solvation being necessary to produce the electronic singlet-based reaction pathway that matches with the observation of para-hydrogen induced polarization in the hydride signals of [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)], W(H)(3)(OH)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) during this study. These studies therefore reveal the existence of differentiated and previously unsuspected thermal and photochemical reaction pathways in the chemistry of both W(N(2))(2)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) which have implications for their reported role in N(2) fixation.

Isomer selective IR-UV depletion spectroscopy of 4-fluorotoluene-NH3: evidence for π-proton-acceptor and linear hydrogen-bonded complexes.

The 4-fluorotoluene-ammonia van der Waals complex has been studied using IR-UV depletion and hole-burning spectroscopies with assignments supported by ab initio and DFT calculations of ground state binding energies and intramolecular vibrational frequencies. The experimental IR-UV depletion and hole-burning spectra presented here provide unequivocal empirical evidence that the 4FT-NH(3) complex exists in two almost equally stable conformational forms. Both isomers contribute intensity to the experimental R2PI spectrum with one responsible for bands appearing to the red of the 4FT band origin position, and the other for those appearing immediately to the blue. On the basis of comparison of computed NH stretching frequencies with those obtained from the IR-UV spectra, the red-shifted bands are assigned to a π-proton-acceptor complex featuring an NH···π-hydrogen bond, and the blue-shifted bands are assigned to an in-plane σ-complex in which the ammonia binds in the plane of the ring forming a double-hydrogen-bonded six-membered ring with the fluorine atom. Ground state interaction energies computed at the M06-2X/cc-pVTZ level were found to compare favourably with those obtained at the CCSD(T) CBS level, although the former resulted in overbinding of the π-complex compared with the in-plane conformer, a characteristic shared with MP2 level calculations. The observation of a π-complex in addition to a σ-complex is consistent with the conclusion that the electron-donating power of the methyl group is sufficiently large to counter-balance the electron-withdrawing power of the fluorine atom.

Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen.

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)(2)(IMes)(py)(3)]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H(2). These reversible processes bring para-H(2) and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in (1)H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)(2)(η(2)-H(2))(IMes)(py)(2)](+), an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach.

The role of the methyl group in stabilising the weak N-H...pi hydrogen bond in the 4-fluorotoluene-ammonia complex.

The 4-fluorotoluene-ammonia van der Waals complex has been studied using a combination of resonant two-photon ionisation (R2PI) spectroscopy, ab initio molecular orbital calculations and multidimensional Franck-Condon analysis. The R2PI spectrum shows two sets of features assignable to two distinct conformers: one in which the ammonia binds between the hydrogen meta to the methyl group and the fluorine atom in a planar configuration and the other a pi-bound structure involving one bond between an ammonia hydrogen and the pi-system and another between the ammonia lone pair and the slightly acidic hydrogens on the methyl group. Ground state estimated CCSD(T) interaction energies were computed at the basis-set limit: these calculations yielded very similar interaction energies for the two conformers, whilst zero point energy correction yielded a zero point binding energy for the pi-complex about 10% larger than that of the in-plane, sigma-complex. The results of multidimensional Franck-Condon simulations based on ab initio ground and excited state geometry optimisations and vibrational frequency calculations showed good agreement with experiment, with further improvements achieved using a fitting procedure. The observation of a pi-complex in addition to a sigma-complex supports the intuitive expectation that electron-donating groups should help to increase pi-density and hence stabilise pi-proton acceptor complex formation. In this case, this occurs in spite of the presence of a strongly electron-withdrawing fluorine atom.

An excited state ab initio and multidimensional Franck-Condon analysis of the A (1)B2 <-- X (1)A1 band system of fluorobenzene.

This work combines high level ab initio calculations with multidimensional Franck-Condon calculations to refine and augment previous assignments of the lower wavenumber region of the A (1)B(2) <-- X (1)A(1) band system of fluorobenzene. The strength of the assignment has been greatly assisted by the use of zero electron kinetic energy spectroscopy in a series of pump-probe experiments where the response of the molecule to selective excitation in specific modes prior to ionization has been studied. The net result of this analysis is the reassignment of 7 of the 12 previously assigned bands in the region below about 1000 cm(-1) using a strategy that aims to trace the origins of excited state normal modes of fluorobenzene to the well-known Wilson modes of benzene by taking full account of the Duschinsky mixing that accompanies electronic excitation. Duschinsky normal mode analyses of the ground and first excited states of fluorobenzene as well as the electronic ground state of fluorobenzene cation have shown that the common use of the benzene Wilson notation to describe normal modes of this prototypical benzene derivative is highly questionable, particularly following electronic excitation and ionization.

An examination of structural characteristics of phenylacetylene by vibronic and rovibronic simulations of ab initio data.

The structural properties of phenylacetylene have been investigated in the S(0)((1)A(1)) neutral ground and S(1)((1)B(2)) and S(2)((1)A(1)) singlet excited states and the D(0)((2)B(1)) cationic state using both rovibronic and multidimensional Franck-Condon simulations from data determined via correlated ab initio methods. Results are compared to experimental and ab initio data reported in the literature. (10,10)-CASSCF and a hybrid CASSCF/SACCI frequency analysis using the cc-pVDZ Dunning basis set have been employed to produce vibronic simulations of REMPI/FES, dispersed fluorescence, TPES and MATI spectra. Calculated rotational constants are used where appropriate to compare to rotationally resolved experimental studies. Whilst the simulations are of generally good quality, it is apparent that the distortion of the ring along the long axis upon electronic excitation is underestimated, resulting in smaller predicted changes in ipso and para CCC bond angles and weaker activities in the 6a and 9a modes compared with experiment. Simulations of one-photon MATI spectra on the other hand, which do not rely on excited state methodologies, compare very well with experiment, suggesting that the neutral and cationic ground state geometries are quite accurate, as are the predicted changes in geometry accompanying ionisation. Simulated rotational and vibrational profiles, as well as other calculated physical data, show good agreement with the numerous experimental and computational studies of phenylacetylene in the literature.

The weak hydrogen bond in the fluorobenzene-ammonia van der Waals complex: Insights into the effects of electron withdrawing substituents on pi versus in-plane bonding.

The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-color resonance enhanced multiphoton ionization (REMPI) spectroscopy, counterpoise corrected RICC2 ab initio molecular orbital calculations, and multidimensional Franck-Condon analysis. The experimental REMPI spectrum is characterized by a dominant, blueshifted band origin, and weak activity in intermolecular vibrational modes. RICC2 geometry optimizations and numerical vibrational frequency calculations of the neutral ground and first excited states have been performed on a number of different structural isomers of the complex using basis sets ranging from augmented double-zeta to quadruple-zeta level. Ground state basis set superposition error corrected zero-point binding energies show the in-plane sigma complex, forming a pseudo-six-membered ring connecting the fluorine atom and ortho-hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable pi and sigma conformers with fluorobenzene show that the sigma complex is the only conformer predicted to exhibit a spectral blueshift upon electronic excitation. The computed neutral ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S(1)-S(0) vibronic spectrum for each of the most stable conformers. These simulations yielded null spectra for transitions involving the most stable of the pi complexes, pi(bridge); a spectrum rich in strong intermolecular vibrational structure for the second of the pi complexes, in complete contrast to the experimental spectrum; and for the sigma complex, a spectrum exhibiting weak intermolecular activity in line with that observed experimentally. This last simulation allowed an almost complete vibrational assignment of the intermolecular structure in the REMPI spectrum. The agreement between computational results and experiment overwhelmingly favors assignment of the spectrum to the in-plane sigma complex.

Photoelectron spectroscopy without photoelectrons: twenty years of ZEKE spectroscopy.

Zero Kinetic Energy (ZEKE) spectroscopy, originally developed as a high resolution form of photoelectron spectroscopy, promised a means to the unambiguous determination of ionic (ro)vibrational states. Since its original development, it has spawned numerous methodological offshoots and has become one of the default methods of choice for high resolution spectroscopy of the ion. This tutorial review describes the historical development of the method, provides some insight into how it works and assesses the impact of the technique by reviewing some of the highlights of the past 20 years as well as some of the more recent developments and applications.

The role of symmetry and optical selection rules in revealing the molecular structure of the lowest Rydberg and ionic states of the 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes.

The 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes (DABCO-Arn) have been investigated using a combination of (1 + 1') resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. The additivity of the spectral shifts observed in both REMPI and ZEKE spectra, taken together with analysis of vibrational structure, suggest that in both DABCO-Ar and DABCO-Ar2 the argon atoms bind in equivalent equatorial (face) locations between two adjacent (CH2)2 bridges. However, the cumulative evidence from both REMPI and ZEKE spectra, together with ab initio results, suggests that the DABCO-Ar3 complex does not revert to D3h symmetry, but rather adopts a C2v structure in which all three argon atoms bind to one side of the DABCO framework. The exceptionally low wave-number vibrational structure observed in the REMPI spectra suggest that the van der Waals interaction in the excited state is extremely weak. However, ionization necessarily increases the strength of the interaction by virtue of the introduction of charge-induced dipole forces, as revealed by a consistent increase in vibrational wave numbers of the modes observed in the resultant ZEKE spectra.

Hydrogenic Rydberg states of molecular van der Waals complexes: resolved Rydberg spectroscopy of DABCO-N2.

The complementary threshold ionization techniques of MATI and ZEKE spectroscopy have been used to reveal well-resolved, long-lived (>10 micros) hydrogenic Rydberg series (50< or =n< or =98) in a van der Waals complex formed between a polyatomic molecule and a diatomic molecule for the first time. The series are observed within 50 cm(-1) of the adiabatic ionization threshold as well as two core-excited thresholds corresponding to excitation of up to two quanta in the van der Waals vibrational mode.