Hydrogenation of pyrrole: Infrared spectra of the 2,3-dihydropyrrol-2-yl and 2,3-dihydropyrrol-3-yl radicals isolated in solid para-hydrogen.

The reaction of hydrogen atoms (H) with pyrrole (C4H4NH) in solid para-hydrogen (p-H2) matrices at 3.2 K has been studied by infrared spectroscopy. Upon reaction of the H atoms with pyrrole in p-H2, a new series of lines appeared in the infrared spectrum, and based on secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species; these lines are designated as set A and set B. According to quantum-chemical calculations performed at the B3PW91/6-311++G(2d,2p) level, the most likely reactions to occur under low temperature conditions in solid p-H2 are the addition of an H atom to carbon 2 or 3 of C4H4NH to produce the corresponding hydrogen-atom addition radicals (HC4H4NH•). When the lines in sets A and B are compared to the scaled harmonic and anharmonic vibrational infrared stick spectra of these two radicals, the best agreement for set A is with the radical produced by the addition to carbon 3 (2,3-dihydropyrrol-2-yl radical, 3-HC4H4NH•), and the best agreement for set B is with the radical produced by addition to carbon 2 (2,3-dihydropyrrol-3-yl radical, 2-HC4H4NH•). The ratio of the 2-HC4H4NH• to 3-HC4H4NH• radicals is estimated to be 4-5:1, consistent with the smaller predicted barrier height for the H-atom addition to C2. In addition to the assignments of the 2,3-dihydropyrrol-2-yl and 2,3-dihydropyrrol-3-yl radicals, a series of lines that appear upon 455-nm photolysis have been assigned to 1,3-pyrrolenine (2-HC4H4N).

Infrared spectra of the 1,1-dimethylallyl and 1,2-dimethylallyl radicals isolated in solid para-hydrogen.

The reaction of hydrogen atoms (H) with isoprene (C5H8) in solid para-hydrogen (p-H2) matrices at 3.2 K has been studied using infrared (IR) spectroscopy. Mixtures of C5H8 and Cl2 were co-deposited in p-H2 at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to produce Cl atoms from the Cl2, and subsequent irradiation with IR light to produce H atoms from the reaction of the Cl atoms with p-H2. The H atoms then react with the C5H8 to form H atom addition radicals (C5H9•). Upon 365-nm/IR photolysis, a multitude of new lines appeared in the IR spectrum and, based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, denoted as set X (an intense line at 776.0 cm-1 and 12 other weaker lines) and set Y (an intense line at 766.7 cm-1 and 12 other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level to determine the relative energetics and predict the IR spectra for the four possible isomers of C5H9• that can be produced from the addition of the H atom to the four distinct carbon atoms in C5H8. The newly observed lines of set X and Y are assigned to the 1,2-dimethylallyl (addition to carbon 4) and the 1,1-dimethylallyl (addition to carbon 1) radicals according to comparison with the predicted IR spectra of the possible products. The 1,2-dimethylallyl radical and the 1,1-dimethylallyl radical were predicted to be the most stable isomers, with the latter ∼8 kJ mol-1 lower in energy than the former and to have significantly lower barriers than the addition pathways for the two central carbons. The ratio of the 1,1-dimethylallyl to the 1,2-dimethylallyl radicals is estimated to be (1.0 ± 0.5):1.0, indicating that the two radicals are produced in similar amounts, which is consistent with the theoretical predictions that the barrier heights are very similar for the H atom addition to the two terminal carbon atoms.

Infrared Spectra of the 1-Chloromethyl-1-methylallyl and 1-Chloromethyl-2-methylallyl Radicals Isolated in Solid para-Hydrogen.

The reaction of chlorine atoms (Cl) with isoprene (2-methyl-1,3-butadiene, C5H8) in solid para-hydrogen (p-H2) matrices at 3.2 K was studied using infrared (IR) spectroscopy. Mixtures of C5H8 and Cl2 were codeposited in p-H2 at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to induce the photodissociation of Cl2 and the subsequent reaction of the Cl atoms with C5H8. Upon 365 nm photolysis, a multitude of new lines appeared in the IR spectrum, and, based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (intense lines at 1237.9, 807.8, and 605.6/608.2 cm-1, and several other weaker lines) and set B (intense lines at 942.4, 1257.7, 796.7/798.5, 667.9, and 569.7 cm-1, and several other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for ·C5H7 and the four possible isomers of the ·C5H8Cl radicals, produced from the addition of the Cl atom to the four distinct sites of carbon atoms in C5H8, to determine the relative energetics and predict IR spectra for each radical. The newly observed lines of sets A and B are assigned to the 1-chloromethyl-2-methylallyl radical (addition to carbon 4) and the 1-chloromethyl-1-methylallyl radical (addition to carbon 1) according to comparison with predicted IR spectra of possible products. The 1-chloromethyl-2-methylallyl radical and 1-chloromethyl-1-methylallyl radicals were predicted to be the most stable, with the latter ∼8 kJ mol-1 lower in energy than the former. The ratio of the 1-chloromethyl-1-methylallyl to the 1-chloromethyl-2-methylallyl radicals is estimated to be (1.2 ± 0.5):1.0, indicating that the two radicals are produced in approximately equal amounts. The exclusive production of the radicals involving the addition of the Cl atom to the two terminal carbons of isoprene is analogous to what was previously observed for the reaction of Cl atoms with trans-1,3-butadiene in solid p-H2.

Experimental and theoretical characterization of a lone pair-π complex: water-hexafluorobenzene.

The lone pair-π interaction between H(2)O and C(6)F(6) was studied using matrix isolation infrared spectroscopy and quantum chemical calculations. Co-deposition of H(2)O with C(6)F(6) in a nitrogen matrix at 17 K followed by annealing to 30 K, results in the appearance of multiple new peaks in the infrared spectrum that are shifted from the H(2)O and C(6)F(6) parent absorptions. These peaks only appear when both the H(2)O and C(6)F(6) are present and have been assigned to distinct structures of a 1:1 H(2)O·C(6)F(6) complex. Similar experiments were performed with D(2)O and HDO and the corresponding infrared peaks for the structures of the D(2)O·C(6)F(6) and HDO·C(6)F(6) complexes have also been observed. Theoretical calculations were performed for the H(2)O·C(6)F(6) complex using the B3LYP, MP2, and CCSD(T) methods. Geometry optimizations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVDZ levels of theory located three structural minima, all of which involve the lone pair-π interaction between the H(2)O and the C(6)F(6) ring, but with different relative orientations of the H(2)O and C(6)F(6) subunits. BSSE corrected interaction energies were estimated at the CCSD(T)/aug-cc-pVTZ level and found to be between -11.2 and -12.3 kJ/mol for the three H(2)O·C(6)F(6) structures. Vibrational frequencies for the each of the structures were calculated at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVDZ levels. The frequencies calculated with both methods support the assignments of the observed new peaks in the infrared spectra to the structures of the H(2)O·C(6)F(6) complex; however, the B3LYP calculated frequency shifts were found to be in better quantitative agreement with the experimentally observed frequency shifts.

Infrared spectrum of the 2-chloroethyl radical in solid para-hydrogen.

The addition reaction of chlorine with ethylene (C(2)H(4)) is expected to proceed via a free radical intermediate, the 2-chloroethyl radical, however, this intermediate has not been previously observed spectroscopically. Irradiation at 365 nm of a co-deposited mixture of Cl(2), C(2)H(4), and p-H(2) at 3.2 K produces a series of new lines in the infrared spectrum. A strong line at 664.0 cm(-1) and weaker lines at 562.1, 1069.9, 1228.0, 3041.1 and 3129.3 cm(-1) are concluded to be due to a single carrier based on their behavior upon subsequent annealing to 4.5 K and secondary irradiation at 254 and 214 nm. The positions and intensities of these lines agree with the MP2/aug-cc-pVDZ predicted vibrational spectrum of the 2-chloroethyl (˙CH(2)CH(2)Cl) radical. In order to confirm this assignment, isotopic experiments were performed with C(2)D(4) and t-C(2)H(2)D(2) and the corresponding infrared bands due to the deuterium isotopomers of this radical (˙CD(2)CD(2)Cl and ˙C(2)H(2)D(2)Cl) have been observed. A final set of experiments were performed following irradiation of the Cl(2)/C(2)H(4)/p-H(2) mixture at 365 nm, in which the matrix was irradiated with filtered infrared light from a globar source, which has been shown to induce reactions between isolated Cl atoms and matrix H(2) to produce HCl and H atoms. In these experiments, the major products observed were HCl, the ethyl radical (˙C(2)H(5)) and ethyl chloride (C(2)H(5)Cl) and the possible mechanisms for the formation of these species are discussed.

Structural variability of pendant groups within the interlayer region of zirconium arene-phosph(on)ates: chemical and structural characterization of oxy- and methyl-linked 2-naphthyl phosphonates, and mixed oxy-linked derivatives.

Several new zirconium phosphonates incorporating the naphthalene ring and having the general formula Zr(O3PR)1(O3PR')1 [R and R' = -CH2C10H7, -OC10H7, -CH3, -OC2H5, -OH] have been synthesized. These materials were chemically characterized using thermal gravimetric analysis (percentage of organic content), infrared spectroscopy (presence of the desired organic functional groups), and solid-state 31P NMR (phosphorus environments), while the structural parameters were determined using X-ray powder diffraction (interlayer d spacings). The two new zirconium bis(phosphonates), Zr(O3PC10H7)2 and Zr(O3PCH2C10H7)2, were found to have d spacings of 19.6 and 20.0 Å, respectively. Three of the four zirconium mixed phosphonates examined are found to be single-phase structures with random mixtures of the organic moieties within the interlayer, and possess d spacings (14.3, 15.3, and 16.1 Å) that are between those of the two parent zirconium bis(phosphonates). The fourth is found to be a staged or segregated structure and possesses a d spacing that is approximately a sum of the two parent zirconium bis(phosphonates), with a d spacing of 28.2 Å. Solid-state 31P NMR of Zr(O3PCH2C10H7)2 revealed the presence of two isotropic resonances, which is interpreted in terms of two distinct, "locked-in" conformations of the -CH2C10H7 group. The experimental d spacings of the zirconium bis(phosphonates) correlate well with a simple predictive model based on the effective length and predominant conformation of the organic functional group.

Matrix isolation infrared observation of HxSi(N2)y (x = 0, 1, 2 and y = 1, 2) transient species using a 121-nm vacuum ultraviolet photolysis source.

Vacuum ultraviolet photolysis (121.6 nm) of silane in a nitrogen matrix at 12 K leads to the observation of several transient species, which have been characterized using Fourier transform infrared spectroscopy. Four transient species containing silicon and nitrogen have been observed (SiN2, Si(N2)2, HSiN2, and H2SiN2), and one transient species containing only silicon and hydrogen has been observed. The assignment of the infrared bands due to each of these species is accomplished by performing isotopic substitution experiments (SiD4, 15N2, and mixtures with SiH4 and 14N2), matrix annealing experiments, UV-visible photolysis experiments, by comparison with previous experimental matrix isolation frequencies, where available, and for HSiN2 and H2SiN2 by comparison to B3LYP/aug-cc-pVTZ-calculated vibrational frequencies. The observation and infrared assignment of the HSiN2 and H2SiN2 molecules in these experiments is significant in that HSiN2 has not been previously reported in the matrix isolation literature, and H2SiN2 has only been reported once previously by a different route of formation. The energetics of the overall formation pathways for the molecules observed in these experiments is discussed using B3LYP/aug-cc-pVTZ calculations.

Quantitative study of interactions between oxygen lone pair and aromatic rings: substituent effect and the importance of closeness of contact.

Current models describe aromatic rings as polar groups based on the fact that benzene and hexafluorobenzene are known to have large and permanent quadrupole moments. This report describes a quantitative study of the interactions between oxygen lone pair and aromatic rings. We found that even electron-rich aromatic rings and oxygen lone pairs exhibit attractive interactions. Free energies of interactions are determined using the triptycene scaffold and the equilibrium constants were determined by low-temperature 1H NMR spectroscopy. An X-ray structure analysis for one of the model compounds confirms the close proximity between the oxygen and the center of the aromatic ring. Theoretical calculations at the MP2/aug-cc-pVTZ level corroborate the experimental results. The origin of attractive interactions was explored by using aromatic rings with a wide range of substituents. The interactions between an oxygen lone pair and an aromatic ring are attractive at van der Waals' distance even with electron-donating substituents. Electron-withdrawing groups increase the strength of the attractive interactions. The results from this study can be only partly rationalized by using the current models of aromatic system. Electrostatic-based models are consistent with the fact that stronger electron-withdrawing groups lead to stronger attractions, but fail to predict or rationalize the fact that weak attractions even exist between electron-rich arenes and oxygen lone pairs. The conclusion from this study is that aromatic rings cannot be treated as a simple quadrupolar functional group at van der Waals' distance. Dispersion forces and local dipole should also be considered.

Ab initio study of substituent effects in the interactions of dimethyl ether with aromatic rings.

Ab initio calculations have been used to investigate the interaction energies of dimers of dimethyl ether with benzene, hexafluorobenzene, and several monosubstituted benzenes. The potential energy curves were explored at the MP2/aug-cc-pVDZ level for two basic configurations of the dimers, one in which the oxygen atom of the dimethyl ether was pointed towards the aromatic ring and the other in which the oxygen atom was pointed away from the aromatic ring. Once the optimum intermolecular distances between the dimethyl and the aromatic ring had been determined for each of the dimers in both configurations at the MP2/aug-cc-pVDZ level, single point energy calculations were performed at the MP2/aug-cc-pVTZ level. A CCSD(T) correction term to the energy was determined and this was combined with the MP2/aug-cc-pVTZ energies to estimate the CCSD(T)/aug-cc-pVTZ interaction energies of the dimers. The estimated CCSD(T)/aug-cc-pVTZ interaction energies are predicted to be attractive for all of the dimers in both configurations and dispersion interactions are found to be a large component of the stabilization of the dimers. For the dimers with the dimethyl ether oxygen pointing towards the aromatic ring, the strengths of interaction energies are found to increase as the aromatic ring becomes more electron deficient, while for the dimers with the dimethyl ether oxygen pointing away from the aromatic ring, they increase as the aromatic ring becomes more electron rich. In both cases, the trends can be explained in terms of the electrostatic potentials of the dimethyl ether and the aromatic rings.

Theoretical Characterization of a Tridentate Photochromic Pt(II) Complex Using Density Functional Theory Methods.

Density functional theory methods have been used to characterize a tridentate photochromic Pt(II) complex [Pt(AAA)Cl], its acetonitrile complex [Pt(AAA)Cl·CH3CN], and the transition state in the complexation reaction. B3LYP/6-31G* (effective core potential for Pt) optimized geometries of Pt(AAA)Cl and Pt(AAA)Cl·CH3CN are found to be in reasonably good agreement with most of the applicable parameters for the available experimental crystal structures of Pt(AAA)Cl and a Pt(AAA)Cl-triphenylphoshine complex, with the exception of one of the dihedral angles, the deviation of which is determined to be due to a steric cis versus trans effect. Vibrational frequencies are calculated for Pt(AAA)Cl and cis-Pt(AAA)Cl·CH3CN, and the predicted shift in the benzaldehyde carbonyl frequency is found to be in line with that observed experimentally. Singlet vertical excitation energies are calculated for Pt(AAA)Cl and cis-Pt(AAA)Cl·CH3CN using time-dependent density-functional theory and are found to be in good agreement with the experimental transition energies, although for cis-Pt(AAA)Cl·CH3CN, the calculations suggest a reassignment of the experimental S1 and S2 transitions. Single point energies are calculated at the B3LYP/6-311+G(2d,2p) level (effective core potential for Pt) and the calculations predict the complexation reaction (dark reaction) to be exothermic and, after a correction to the entropy, to be exoergic at 298 K and to proceed with a reasonable activation energy. Based on singlet and triplet vertical excitation energies, it is speculated that the photoreaction occurs via an intersystem crossing from S1 to T1 for cis-Pt(AAA)Cl·CH3CN followed by an adiabatic reaction along the T1 surface and then nonradiative intersystem crossing to the S0 state of Pt(AAA)Cl.

Substituent effects in C6F6-C6H5X stacking interactions.

Parallel displaced and sandwich configurations of hexafluorobenzene-substituted benzene dimers are studied by ab initio molecular orbital methods up to the MP2(full)/aug-cc-pVDZ level of theory to reveal how substituents influence pi-pi interactions. Two minimum energy configurations are found, one with the substituent group away from the pi-face of the hexafluorobenzene ring (2a-f) and the other with the substituent group on top of the pi-face of the hexafluorobenzene (3a-f). Higher binding energies are predicted for dimers with the substituent on the pi-face (3a-f). All sandwich dimers (4a-e) are found to be saddle points on the potential energy surfaces. A parallel-displaced minimum energy configuration is also predicted for the parent complex, C6F6-C6H6, which is in contrast to predictions based on quadrupole moments of benzene and hexafuorobenzene. The preference for the parallel displaced, rather than the sandwich configuration, is rationalized based on the smaller interplanar distance in the former. The closeness of contact in the parallel-displaced dimers leads to greater binding energies. The shape of the electron density isosurface of the monomers is suggested to provide a guide for the prediction of how arenes stack with one another. A large difference in binding energy between the C6F6 complex of aniline (3e) and N,N-dimethylaniline (3f) is calculated, and charge-transfer interactions are suggested to play a role in the latter.

Theoretical study of the benzene excimer using time-dependent density functional theory.

A theoretical characterization of the potential energy surfaces of the singlet benzene excimer states derived from the B2u monomer excited state has been performed using time-dependent density functional theory. The excited-state potential energy surfaces were initially characterized by computations along the parallel and perpendicular intermolecular translational coordinates. These calculations predict that the lowest excited state for parallel translation is bound with a minimum at 3.15 angstroms and with a binding energy of 0.46 eV, while the perpendicular translational coordinate was essentially found to be a repulsive state. At the calculated minimum distance, the effect of in-plane rotation, out-of-plane rotation, and slipped-parallel translation were examined. The rotational calculations predict that deviations from the D6h geometry lead to a destabilization of the excimer state; however, small angular variations in the range of 0 degrees -10 degrees are predicted to be energetically feasible. The slipped-parallel translational calculations also predict a destabilizing effect on the excimer state and were found to possess barriers to this type of dissociation in the range of 0.50-0.61 eV. When compared to experimentally determined values for the benzene excimer energetics, the calculated values were found to be in semiquantitative agreement. Overall, this study suggests that the time-dependent density functional theory method can be used to characterize the potential energy surfaces and the energetics of aromatic excimers with reasonable accuracy.

Zirconium arene-phosphonates: chemical and structural characterization of 2-naphthyl- and 2-anthracenylphosphonate systems.

Several new zirconium phosphonates incorporating naphthalene and anthracene ring systems and having the general formula Zr(O3PR)1(O3PR')1 [R and R' = -C10H7, -C14H9, -OC4H9, and -OC2H5] have been synthesized. These compounds were chemically characterized using thermal gravimetric analysis (percentage of organic content), infrared spectroscopy (presence of the desired organic functional groups), and solid-state 31P NMR (phosphorus environments), while the structural parameters were determined using X-ray powder diffraction (interlayer d spacings). The d spacings of the zirconium bis(phosphonates) correlate well with a simple predictive model based on the effective length of the organic functional group. The zirconium mixed phosphonates examined are single-phase structures with random mixtures of the organic moieties within the interlayer and possess d spacings that are between those of the two parent zirconium bis(phosphonates).

Excimer formation in the interlayer region of arene-derivatized zirconium phosphonates.

Photophysical investigations are reported for two arene-derivatized zirconium phosphonates (layered solids). The chromophore groups that are attached within the interlayer region of these materials are seen to form excimer pairs. Whereas the naphthalene-containing system exhibits both monomer and excimer fluorescence, the potentially greater overlap of chromophores for the system containing anthracene-pendant groups causes that compound to only show excimer fluorescence.