Resonance Raman and density functional theory investigation of the photodissociation dynamics of the A-band absorption of (E)-beta-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 study of short-time photodissociation dynamics of the charge-transfer band absorption of nitrobenzene in cyclohexane solution

Resonance Raman spectra were obtained for nitrobenzene 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 mainly along the nominal NO2 symmetric stretch mode (nu 11), the nominal benzene ring stretch mode (nu 7), accompanied by a moderate degree of motion along the nominal ONO symmetry bend/benzene ring stretch mode (nu 23), the nominal C-N stretch/benzene ring breathing mode (nu 16), the nominal CCC bending mode (nu 20) and the nominal CCH in-plane bending mode (nu 14). A preliminary resonance Raman intensity analysis was done and the results for nitrobenzene were compared to previously reported results for several nitroalkanes.     

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

Resonance Raman spectra were obtained for 2-hydroxybenzaldehyde (OHBA) in cyclohexane solution with excitation wavelengths in resonance with the first charge-transfer/proton-transfer (CT/PT) band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion predominantly along the nominal C=CH in-plane bend+ring deformation modes (nu9, nu10, nu14, nu16, nu18, nu19, nu20, nu26, nu30, nu31, and nu35) accompanied by a smaller amount of motion along the nominal C=O stretch mode (nu7), the nominal C=C-C(=O) in-plane bend modes (nu33 and nu37), and the nominal ring C-O-H in-plane bend modes (nu9 and nu14). A preliminary resonance Raman intensity analysis was done, and these results for the OHBA molecule were compared to results previously reported for the 2-hydroxyacetophenone (OHAP) molecule. Several proton-transfer tautomers in the ground and excited states were predicted from the results of B3LYP/cc-PVTZ, UB3LYP/cc-PVTZ, and CASSCF/cc-PVDZ level of theory computations. The differences and similarities between the CT/PT band resonance Raman spectra and the vibrational reorganizational energies for the OHBA molecule relative to those for the OHAP molecule are briefly discussed.     

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.     

Resonance Raman and theoretical investigation of the photodissociation dynamics of benzamide in S3 state

Resonance Raman spectra were obtained for benzamide in methanol and acetonitrile solutions with excitation wavelengths in resonance with the S(3) state. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with the motions mainly along the benzene ring C[Double Bond]C stretch nu(9), the Ph-CO-NH(2) and ring benzene stretch nu(14), the CCH in plane bend nu(17), the Ph-CO-NH(2) stretch and NH(2) rock nu(19), the ring trigonal bend nu(23), and the ring deformation and Ph-CO-NH(2) stretch nu(29). A preliminary resonance Raman intensity analysis was done, and the results were compared to those previously reported for acetophenone to examine the substituent effect. Solvent effect on the short-time photodissociation dynamics of benzamide was also examined. A conical intersection point S(2)S(3) between S(3) and S(2) potential energy surfaces of benzamide was determined by using a complete active space self-consistent field theory computations. The structural differences and similarities between S(3)S(2) point and S(0) were examined, and the results were used to correlate to the Franck-Condon photodissociation dynamics of benzamide in S(3) state.     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

Imaging the state-specific vibrational predissociation of the C2H2-NH3 hydrogen-bonded dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions.     

Photoinitiated predissociation of the NO dimer in the region of the second and third NO stretch overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

Quantum tunneling in the midrange vibrational fundamentals of tropolone

The Fourier transform infrared spectrum of tropolone(OH) vapor in the 1175-1700 cm(-1) region is reported at 0.0025 and 0.10 cm(-1) spectral resolutions. The 12 vibrational fundamentals in this region of rapidly rising vibrational state density are dominated by mixtures of the CC, CO, CCH, and COH internal coordinates. Estimates based on the measurement of sharp Q branch peaks are reported for 11 of the spectral doublet component separations DS(v) = |Delta(v) +/- Delta(0)|. Delta(0) = 0.974 cm(-1) is the known zero-point splitting, and three a(1) modes show tunneling splittings Delta(v) approximately Delta(0), four b(2) modes show splittings Delta(v) approximately 0.90Delta(0), and the remaining four modes show splittings Delta(v) falling 5-14% from Delta(0.) Significantly, the splitting for the nominal COH bending mode nu(8) (a(1)) is small, that is, 10% from Delta(0). Many of the vibrational excited states demonstrate strong anharmonic behavior, but there are only mild perturbations on the tautomerization mechanism driving Delta(0). The data suggest, especially for the higher frequency a(1) fundamentals, the onset of selective intramolecular vibrational energy redistribution processes that are fast on the time scale of the tautomerization process. These appear to delocalize and smooth out the topographical modifications of the zero-point potential energy surface that are anticipated to follow absorption of the nu(v) photon. Further, the spectra show the propensity for the Delta(v) splittings of b(2) and other complex vibrations to be damped relative to Delta(0).     

Vibrational normal modes of diazo-dimedone: a comparative study by Fourier infrared/Raman spectroscopies and conformational analysis by MM/QM

The 2-diazo-5,5-dimethyl-cyclohexane-1,3-dione (3) was synthesized and the FT-IR/Raman spectra were measured with the purpose of obtain a full assignment of the vibrational modes. Singular aspects concerning the -CNN oscillator are discussed in view of two strong bands observed in the region of 2300-2100 cm(-1) in both, Infrared and Raman spectra. The density functional theory (DFT) was used to obtain the geometrical structure and for assisting in the vibrational assignment joint to the traditional normal coordinate analysis (NCA). The observed wavenumbers at 2145 (IR), 2144(R) are assigned as the coupled nu(NN)+nu(CN) vibrational mode with higher participation of the NN stretching. A 2188 cm(-1) (IR) and at 2186 cm(-1) (R) can be assigned as a overtone of one of nu(CC) normal mode or to a combination band of the fundamentals delta(CCH) found at 1169 cm(-1) and the delta (CCN) found at 1017 cm(-1) enhanced by Fermi resonance.     

Intersite structural rearrangement of the blue copper site induced by substrate binding: spectroscopic studies of a copper-containing nitrite reductase from Alcaligenes xylosoxidans NCIMB 11015

A copper-containing nitrite reductase from Alcaligenes xylosoxidans NCIMB 11015 has its own unique blue or type 1 copper protein resonance Raman spectrum in the usual Cu-S(Cys) stretching region, nu(Cu-S(Cys)), with a pair of strong peaks at 412 and 420 cm(-1) and a weak peak at 364 cm(-1). The predominantly nu(Cu-S(Cys)) Raman bands at 412, 420, and 364 cm(-1) of the type 1 copper site all shifted to higher frequencies upon binding of nitrite to the type 2 copper site, and the resonance Raman difference spectra progressively intensified with the increments of nitrite ion concentration. Positive support for substrate binding to the type 2 copper is provided by the nu(Cu-S(Cys)) bands in the resonance Raman spectrum of a type 2 copper-depleted enzyme, which is insensitive to the presence of NO2-. The shift to higher frequency of the Raman bands of the type 1 copper center with the addition of nitrite ions suggests a stronger Cu-S(Cys) interaction in the substrate-bound A. xylosoxidans nitrite reductase.     

Structure and dynamics of the solvated electron in alcohols from resonance Raman spectroscopy

Resonance Raman (RR) spectroscopy is used to probe the structure and excited-state dynamics of the solvated electron in the primary liquid alcohols methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH). The strong resonance enhancements (>or=10(4) relative to pure solvent) of the libration, CO stretch, COH bend, CH3 bend, CH2 bend, and OH stretch reveal significant Franck-Condon coupling of the intermolecular and intramolecular vibrational modes of the solvent to the electronic excitation of the solvated electron. All enhanced bands are fully accounted for by a model of the solvated electron that is comprised of several nearby solvent molecules that are only perturbed by the presence of the electron; no new molecular species are required to explain our data. The 340 cm(-1) downshift observed for the OH stretch frequency of e-(MeOH), relative to pure solvent, strongly suggests that the methanol molecules in the first solvent shell have the hydroxyl group directed linearly toward the excess electron density. The smaller downshifts observed for e-(EtOH), e-(n-PrOH), and e-(n-BuOH) are explained in terms of a OH group that is bent 28-40 degrees from linear. The Raman cross sections and absorption spectra are modeled, lending quantitative support for the inhomogeneous origin of the broad absorption spectra, the necessity of OH local motion in all enhanced Raman modes of the alcohols, and the dominant librational response of the solvent upon photoexcitation of the electron.     

Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 A) and krypton (6764 A) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm(-1), assigned to ring-metal-ring stretching, ring-metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C-H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)2-, and thorocene. All vibrational frequencies of the ligands, except the 259 cm(-1) out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about approximately 460 cm(-1) was also observed. The 460 cm(-1) band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm(-1) with 6764 A excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra.     

Energy flow in push-pull chromophores: vibrational dynamics in para-nitroaniline.

para-Nitroaniline (PNA) plays an essential role as the prototype model of push-pull chromophores. The nature and degree of participation of vibrational degrees of freedom in the charge-transfer and internal-conversion processes are current issues of great theoretical and practical importance. Ultrafast time-resolved anti-Stokes resonance Raman spectroscopy (TRARRS) experiments on PNA in dimethyl sulfoxide with three different excitation wavelengths were performed to probe these dynamical influences. The vibrational dynamics associated with S0 were independent of incident wavelength, and this supports the picture that the S1 dynamics are fast relative to the rate of intersystem crossing. The phenyl breathing mode nu(19) (860 cm(-1)) and the symmetric NO2 stretch nu(29) (1310 cm(-1)) exhibited vibrational lifetimes in S0 of 8.1 and 5.2 ps, respectively. No evidence for inhomogeneous broadening of the charge-transfer band in the UV/Vis absorption spectrum was found.     

Vibrational frequencies and structural determination of tetraethynylstannane

The normal mode frequencies and corresponding vibrational assignments of Sn(CCH)₄ are examined theoretically using the 98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of six types of motion predicted by a group theoretical analysis (Sn–C stretch, CC stretch, C–H stretch, CC–H bend, Sn–CC bend, and C–Sn–C bend) utilizing the T_d symmetry of the molecule. A set of uniform scaling factors were derived for each type of motion. Predicted infrared and Raman intensities are reported.     

Infrared spectroscopy of gas phase C3H5+ : the allyl and 2-propenyl cations

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800-3500 cm(-1) region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C(2nu) charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a C(s) symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group.     

Observation of nu1+nu(n) combination bands of the HOOO and DOOO radicals using infrared action spectroscopy

Hydrogen trioxy (HOOO) and its deuterated analog (DOOO) have been generated in a supersonic free-jet expansion through association of photolytically generated OH or OD and molecular oxygen. The radicals were detected using infrared action spectroscopy, a highly sensitive double resonance technique. Rotationally resolved spectra of combination bands of HOOO and DOOO comprising one quantum of OH or OD stretch (nu(1)) and one quantum of a lower frequency mode (nu(1)+nu(n) where n=3-6), including HDOO bend (nu(3)), OOO bend (nu(4)), central OO stretch (nu(5)), and HDOOO torsion (nu(6)), have been observed and assigned to the trans conformer. All but one of these bands are accompanied by unstructured features which are tentatively assigned to the corresponding vibration of the cis conformer. In total, five additional bands of HOOO and four of DOOO have been recorded and assigned. These data represent the first gas-phase observation of the low-frequency modes of HOOO and DOOO and they are found to differ significantly from previous matrix studies and theoretical predictions. Accurate knowledge of the vibrational frequencies is crucial in assessing thermochemical properties of HOOO and present possible means of detection in the atmosphere.     

Infrared overtone spectroscopy and vibrational analysis of a Fermi resonance in nitric acid: Experiment and theory

High resolution infrared spectra of nitric acid have been recorded in the first OH overtone region under jet-cooled conditions using a sequential IR-UV excitation method. Vibrational bands observed at 6933.39(3), 6938.75(4), and 6951.985(3) cm(-1) (origins) with relative intensities of 0.42(1), 0.38(1), and 0.20(1) are attributed to strongly mixed states involved in a Fermi resonance. A vibrational deperturbation analysis suggests that the optically bright OH overtone stretch (2nu1) at 6939.2(1) cm(-1) is coupled directly to the nu1 + 2nu2 state at 6946.4(1) cm(-1) and indirectly to the 3nu2 + nu3 + nu7 state at 6938.5(1) cm(-1). Both the identity of the zero-order states and the indirect coupling scheme are deduced from complementary CCSD(T) calculations in conjunction with second-order vibrational perturbation theory. The deperturbation analysis also yields the experimental coupling between 2nu1 and nu1 + 2nu2 of -6.9(1) cm(-1), and that between the two dark states of +5.0(1) cm(-1). The calculated vibrational energies and couplings are in near quantitative agreement with experimentally derived values except for a predicted twofold stronger coupling of 2nu1 to nu1 + 2nu2. Weaker coupling of the strongly mixed states to a dense background of vibrational states via intramolecular vibrational energy redistribution is evident from the experimental linewidths of 0.08 and 0.25 cm(-1) for the higher energy and two overlapping lower energy bands, respectively. A comprehensive rotational analysis of the higher energy band yields spectroscopic parameters and the direction of the OH overtone transition dipole moment.