Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane

Taking the plunge: chemical reaction dynamics in liquids

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S4(21𝝅𝝅*) state

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S₄(2¹ππ^*) light-absorbing state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The B-band absorption cross-section and the corresponding absolute resonance Raman cross-sections were simulated using a simple model based on time-dependent wave-packet theory. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S₄(2¹ππ^*)/S₃(2¹nπ^*)-MIN, S₄(2¹ππ^*)/S₁(1¹nπ^*)-MIN, and S₄(2¹ππ^*)-MIN. Our results indicate that the initial population of NHP in the S₄ state bifurcates in or near the Franck-Condon region, leading to two predominant (S₄S₃-MIN and S₄S₁-MIN) internal conversion pathways. The lowest-lying S₂(1¹ππ^*) excited state is finally formed via subsequent internal conversions S₃(2¹nπ^*)/S₂(1¹ππ^*)-MIN and S₁(1¹nπ^*)/S₂(1¹ππ^*)-MIN. The enol-keto tautomeric mechanism does not seem to play a role. The decay mechanism in the singlet realm is proposed.

Decay dynamics of α,β-carboxylic methyl esters (CH3OCOCH:CHR) in the lower-lying excited states--resonance Raman and complete active space self-consistent field calculation study

The photophysics of two α,β-carboxylic methyl esters after excitation to the light absorbing S2(ππ(*)) state were studied by using the resonance Raman spectroscopy and complete active space self-consistent field (CASSCF) method calculations. The vibrational spectra were assigned on the basis of the experimental measurements and the B3LYP/6-31G(d) computations, as well as the normal mode analysis. The A-band resonance Raman spectra of methyl 2,4-pentadienoate (M24PDA) and methyl trans cronoate (MTCA) were measured to probe the structural dynamics in Franck-Condon region. CASSCF calculations were done to obtain the minimal excitation energies and geometric structures of the lower-lying singlet and triplet excited states, and the curve-crossing points. It was revealed that the short-time structural dynamics of M24PDA was dominated by the Cα=Cβ-C4=C5 stretch coordinate, while that of MTCA was mostly along the Cα=Cβ and the C=O stretch motion. Comparison of the structural dynamics of M24PDA and MTCA with that of 3-methyl-3-pentene-2-one (3M3P2O) indicated that the structural dynamics of MTCA is similar to that of 3M3P2O but different than that of M24PDA in that the variation of the Raman intensity ratios for ν7/ν8, (ν7+ν8)/2ν8, (ν7+2ν8)/3ν8, (ν7+3ν8)/4ν8 of MTCA is similar to that of 3M3P2O but different from that of M24PDA. It is found that the substitution of methyl group in the α(')-position of α,β-enones by methoxyl group does not substantially affect the short-time structural dynamics, while the substitution of vinyl group in the β-position changes significantly the short-time structural dynamics and the subsequent decay processes. A detailed decay mechanism is proposed. Two sub-processes which consider the reconjugation and the subsequent charge-transfer reaction of O=C-Cα=Cβ chromophore were postulated to describe the variation of short-time structural dynamics with the different substitution.

Resonance Raman and density functional theory investigation of the photodissociation dynamics of the A-band absorption of (E)-β-nitrostyrene in cyclohexane solution

Resonance Raman spectra were obtained for (E)-beta-nitrostyrene in cyclohexane solution with excitation wavelengths in resonance with the charge transfer (CT)-band absorption spectrum. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion predominantly along the nominal NO(2) symmetric stretch mode (nu(14)), the nominal C=C stretch mode (nu(8)), the nominal benzene ring stretch mode (nu(9)), accompanied by a smaller amount of motion along the nominal ONO symmetric bend/benzene ring stretch mode (nu(34)), the nominal CCH in-plane bending mode (nu(20)), the nominal HC=CH in-plane bending mode (nu(18)), the nominal NO(2) asymmetric stretch mode (nu(11)), the nominal C-N stretch/benzene ring breathing mode (nu(27)), and the nominal CCC trigonal bending mode (nu(25)). A preliminary resonance Raman intensity analysis was done and these results for (E)-beta-nitrostyrene were compared to results previously reported for several nitrobenzene and trans-stilbene compounds. The differences and similarities between the CT-band resonance Raman spectra and vibrational reorganizational energies for (E)-beta-nitrostyrene relative to those for nitrobenzene and trans-stilbene were briefly discussed.

Resonance Raman intensity analysis of the excited state proton transfer dynamics of 2-nitrophenol in the charge-transfer band absorption

Resonance Raman spectra were obtained for 2-nitrophenol in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) proton transfer band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion along more than 15 normal modes: the nominal CCH bend+CC stretch nu(12) (1326 cm(-1)), the nominal CCC bend nu(23) (564 cm(-1)), the nominal CO stretch+NO stretch+CC stretch nu(14) (1250 cm(-1)), the nominal CCH bend+CC stretch+COH bend nu(15) (1190 cm(-1)); the nominal CCH bend+CC stretch nu(17) (1134 cm(-1)), the nominal CCC bend+CC stretch nu(22) (669 cm(-1)), the nominal CCN bend nu(27) (290 cm(-1)), the nominal NO(2) bend+CC stretch nu(21) (820 cm(-1)), the nominal CCO bend+CNO bend nu(25) (428 cm(-1)), the nominal CC stretch nu(7) (1590 cm(-1)), the nominal NO stretch nu(8) (1538 cm(-1)), the nominal CCC bend+NO(2) bend nu(20) (870 cm(-1)), the nominal CC stretch nu(6) (1617 cm(-1)), the nominal COH bend+CC stretch nu(11) (1382 cm(-1)), nominal CCH bend+CC stretch nu(9) (1472 cm(-1)). A preliminary resonance Raman intensity analysis was done and the results for 2-nitrophenol were compared to previously reported results for nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone. The authors briefly discuss the differences and similarities in the CT-band absorption excitation of 2-nitrophenol relative to those of nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone.

Resonance Raman Investigation of the Short-Time Photodissociation Dynamics of the Charge-Transfer Absorption of the I2−Benzene Complex in Benzene Solution

Resonance Raman spectra were obtained for the I2-benzene complex in benzene solvent with excitation wavelengths in resonance with the CT-band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal I-I stretch mode nu(18), the nominal symmetric benzene ring stretch mode nu5, and the nominal symmetric CCH bending nu7. There is also a small contribution from the nominal out-of-plane CH oop wag nu15. A preliminary resonance Raman intensity analysis was done, and the results for the I2-benzene complex were compared to results previously reported for the 1-hexene-I2 complex. We briefly discuss the differences and similarities in the CT-band absorption excitation of an I2-benzene complex relative to those of an I2-alkene complex.