Effects of torsion on O-H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.     

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.     

Quantum chemical study of chlorine-dissociation of oxalyl chloride (ClCO)_2→2Cl+2CO

It is very difficult to study the phenomenon that molecules are decomposed into several pho-tofragments by UV light, as the energy of lamp-house is insufficient. But the bond energy in oxa-lyl chloride is relatively low, for example, D0(ClCOCO-Cl) = 313.92 kJ/mol[1], D0(ClCO-CO) = 35.53 kJ/mol, and D0(Cl-CO) = 27.17 kJ/mol[2], so, oxalyl chloride, as a typical system for the study of multi-channel dissociation, can be dissociated into the four fragments Cl+Cl+CO+CO by the proper UV …     

Spectroscopy, dissociation dynamics, and potential energy surfaces for CN(A)-Ar

The A (2)Pi-X (2)Sigma(+) band system of CN-Ar has been examined using fluorescence depletion and action spectroscopy techniques. Eight vibronic bands of the complex were observed in association with the monomer 3-0 transition. Pump-probe measurements were used to characterize CN(A (2)Pi(32),nu=3) fragments from direct photodissociation of CN(A (2)Pi,nu=3)-Ar and CN(X (2)Sigma(+),nu=7) fragments from CN(A (2)Pi,nu=3)-Ar predissociation. The latter showed a marked preference for population of positive parity diatomic rotational levels. Bound state calculations were used to assign the A-X bands and to obtain fitted potential energy surfaces for the A state. The average potential obtained from fitting had a well depth of D(e)=137.8 cm(-1). High-level ab initio calculations were used to obtain equilibrium Jacobi coordinates of theta(e)=94 degrees and R(e)=7.25 bohr. The near-symmetric character of the fitted potential energy surface was consistent with the symmetry preference observed in the predissociation dynamics.     

Laser-induced fluorescence spectra, structure, and the ring-twisting and ring-bending vibrations of 1,4-benzodioxan in its S0 and S1(pi,pi*) states

The laser-induced fluorescence (LIF) spectra, both the fluorescence excitation spectra (FES) and single vibrational level fluorescence spectra (SVLF) from several different vibronic states, along with the ultraviolet (UV) absorption spectra of 1,4-benzodioxan have been recorded and analyzed. A detailed energy map has been constructed for four low-frequency vibrations and their combinations for both the S(0) and S(1)(pi,pi) electronic states. These are nu(48) (ring-bending), nu(25) (ring-twisting), nu(47) (ring-flapping), and nu(24) (skeletal-twisting). Both the experimental and ab initio calculations show the molecule to be twisted in both the S(0) and S(1)(pi,pi) states with high barriers to planarity. The experimentally determined ring-twisting quantum states, which are confined to the lower regions of the potential energy surface, were used to calculate one-dimensional potential functions in terms of the twisting coordinates, and the extrapolated barriers were estimated to be 5700 and 4200 cm(-1) for the S(0) and S(1) states, respectively. Two-dimensional calculations, which included the interactions with the bending modes, gave values of 3906 and 1744 cm(-1), respectively. The S(0) value compares favorably with the ab initio value of 4095 cm(-1).     

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.     

Infrared spectra and relative stability of the F3CH/NH3 H-bonded complex in liquefied Xe

FTIR spectra of mixtures of fluoroform (F3CH) and ammonia (NH3), have been studied in liquid xenon between 5400 and 500 cm-1. Spectroscopic evidence for the formation of a hydrogen bonded complex has been found and the complexation enthalpy Delta(LXe)H degrees in the temperature interval between 173 and 215 K, was determined to be 14.4 (7) kJ mol-1. The parallel fundamentals nu1 and nu2 of ammonia reveal a strong narrowing effect upon complex formation, whereas the perpendicular fundamentals nu3 and nu4 show a modest decrease of their width. CP corrected ab initio calculations at the MP2(FULL)/6-311++G(3df,2pd) level predict a linear geometry for the complex, characterized by a small red shift of the CH stretch frequency of fluoroform. The ab initio interaction energy was found to be compatible with the isolated molecule complexation energy extrapolated from the experimental Delta(LXe)H degrees .     

The ground state energy levels and molecular structure of jet-cooled HGeCl and DGeCl from single vibronic level emission spectroscopy

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl have been recorded by laser excitation of selected bands of the A 1A"-X 1A' electronic transition. Twenty-six ground state vibrational levels of HGeCl and 42 of DGeCl were measured, assigned, and fitted to standard anharmonicity expressions, which allowed all the harmonic frequencies to be determined for both isotopomers. A normal coordinate least squares analysis obtained by fitting the harmonic frequencies yielded reliable values for five of the six force constants. The ground state effective rotational constants and force field data were combined to calculate average (rz) and approximate equilibrium (re z) structures, with re z(GeH)=1.586(1) A, re z(GeCl)=2.171(2) A, and the bond angle fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.9 degrees . Comparisons show that the derived bond lengths are consistent with those of the appropriate diatomic molecules in their ground electronic states and the bond angle is similar to that of germylene (GeH2). A Franck-Condon simulation of the vibrational intensities in the 0(0) (0) band emission spectrum of HGeCl using ab initio force field data shows good agreement with experiment, lending credence to the vibrational analysis of the observed spectra.     

A family of new boron-containing free radicals

The first spectroscopic evidence for the existence of the HBX and DBX (X = F, Cl, Br) free radicals has been obtained from laser-induced fluorescence and wavelength-resolved emission spectra. The vibrational frequencies measured in emission are in excellent agreement with our ab initio (CCSD(T)/aug-cc-pVTZ) predictions. The patterns of resolved rotational sub-band structure and deuterium isotope effects have been used to positively identify the radicals. The experimental data and ab initio predictions will be useful in searches for the matrix infrared and microwave spectra of these new radicals.     

Experimental and theoretical studies of the CN-Ar van der Waals complex

The CN-Ar van der Waals complex has been observed using the B (2)Sigma(+)-X (2)Sigma(+) and A (2)Pi-X (2)Sigma(+) electronic transitions. The spectra yield a dissociation energy of D(0")=102+/-2 cm(-1) and a zero-point rotational constant of B(0")=0.067+/-0.005 cm(-1) for CN(X)-Ar. The dissociation energy for CN(A)-Ar was found to be D(0')=125+/-2 cm(-1). Transitions to vibrationally excited levels of CN(B)-Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces (PESs) for the X and B states. Ab initio PESs were calculated for the X and B states. These were used to predict rovibrational energy levels and van der Waals bond energies (D(0")=115 and D(0')=183 cm(-1)). The results for the X state were in reasonably good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment. Electronic predissociation was observed for both CN(A)-Ar and CN(B)-Ar. The process leading to the production of CN(A,nu=8,9) fragments from the predissociation of CN(B,nu=0)-Ar was characterized using time-resolved fluorescence and optical-optical double resonance measurements.     

High-resolution FTIR, microwave, and ab initio investigations of CH2 79BrF: ground, v(5) = 1, and v(6) = 1, 2 state constants

A spectroscopic study of CH279BrF in the infrared and microwave regions has been carried out. The rovibrational spectrum of the nu5 fundamental interacting with 2nu6 has been investigated by high-resolution FTIR spectroscopy. Owing to the weakness of the 2nu6 band, the v6 = 2 state constants have been derived from v6 = 1. For this reason, the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. Highly accurate ab initio computations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation. As far as the nu5 band is concerned, the analysis of the rovibrational structure led to the identification of more than 3000 transitions, allowing the determination of a set of spectroscopic parameters up to sextic distortion terms and pointing out first-order c-type Coriolis interaction with the v6 = 2 state. With regard to the pure rotational spectra measurements, the assignment of several DeltaJ = 0, +1 transitions allowed the determination of the rotational, all the quartic, and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole coupling tensor for both the ground and v6 = 1 states.     

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.     

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.     

Calculation of the vibronic structure of the X2Pi photoelectron spectra of XCN, X=F, Cl, and Br

The vibronic structure of the photoelectron spectra of the X (2)Pi state of XCN(+) (X=F, Cl, and Br) has been calculated, assuming that the X (2)Pi state can be considered as an isolated electronic state. The Renner-Teller coupling of the two components of the (2)Pi state via the degenerate bending mode as well as spin-orbit coupling effects are taken into account. The two stretching modes are treated within the so-called linear vibronic-coupling model. The vibronic and spin-orbit parameters have been determined by accurate ab initio electronic-structure calculations. While spin-orbit effects are small in FCN(+), the large spin-orbit splitting of the X (2)Pi state of the BrCN(+) leads to a complete quenching of the Renner-Teller effect. The X (2)Pi state of the ClCN(+) is shown to be of particular interest: here the resonance condition for linear-relativistic Renner-Teller coupling is approximately fulfilled. This coupling mechanism leads to a significant intensity transfer to vibronic levels with odd quanta of the bending mode. The calculated spectrum indicates that this novel relativistic vibronic-coupling effect should be observable in high-resolution (electron energy resolution of the order of a few meV) photoelectron spectra of ClCN.     

Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v"=0), with low rotational excitation. The rotational distribution of OH (v"=0,J) is characterized by a temperature of 433+/-31 K, corresponding to a rotational energy of 0.86+/-0.06 kcalmol. Two Lambda-doublet levels, 2Pi+(A') and 2Pi-(A"), and the two spin-orbit states, the 2Pi(3/2) and 2Pi(1/2), of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9+/-3.2 kcal mol(-1), from the Doppler-broadened linewidth, giving an ft value of approximately 43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40 kcal mol(-1). The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C-O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C-OH bond cleavage giving OH.     

Reactions of C(1D) with H2 and its deuterated isotopomers, a wave packet study

Using a Chebyshev wave packet method, total and state-resolved reaction probabilities (J=0) were calculated for the reactions of C(1D) with various hydrogen isotopomers (H2, D2, and HD, nu i=0, j i=0) on a recent ab initio potential energy surface. For all the isotopic variants, it was found that the initial state specified reaction probabilities have no energy threshold and are strongly oscillatory, indicative of the involvement of long-lived resonances in this barrierless reaction. The J=0 product vibrational and rotational distributions for all three isotopic reactions, and the CH/CD branching ratio for the C+HD reaction, show strong dependence on the collision energy, further underscoring the important role played by the resonances. The generally decaying vibrational distributions and highly excited rotational distributions, which corroborate an insertion mechanism, and the dominance of the CD+H channel in the C+HD reaction are consistent with existing experimental observations. Initial state specified integral cross sections and rate constants were estimated using a capture model. The estimated rate constants were found to be close and in the order kHD>kH2>kD2. Finally, a method to calculate branching ratio in the C+HD reaction is proposed.     

Mass-analyzed threshold ionization spectroscopy of pyridine. Structural distortion in the first excited state

For the first time, vibrational spectra of the pyridine cation in the electronic ground state have been measured via several intermediate states (0(0), 16b0(2), 16b0(4), 6a0(1), 6b(1), 16a0(1), 10a0(1) and 12(1)) by Mass-Analyzed Threshold Ionization (MATI) spectroscopy. From the MATI spectra, the adiabatic ionization energy of pyridine has been determined to be 74,185 +/- 6 cm(-1) (9.1978 +/- 0.0008 eV). Several vibronic modes in the ionic ground state could be assigned for the first time. An intensity gain of vibrations having b1 symmetry could be observed by activating the ion ground state. Also, a breakdown of the "delta nu = 0 propensity rule" for the excitation via the 16b(2) and 16b(4) states of the first excited states are displayed in the recorded spectra. In conjunction with ab initio calculations these observations can be explained by a strong geometrical distortion along the 16b vibration in the first excited state, leading to a "boat distortion".     

Gas phase electronic spectrum of linear AlCCH

The electronic spectrum of the aluminium containing species AlCCH has been detected in the gas phase in the region 315-355 nm. The experiment used a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Structures of the AlCCH isomers have been optimized using density functional theory (DFT) and the excitation energies to the low-lying electronic excited states calculated. Based on the analysis of the observed rotational structure and the theoretical data, the spectrum is assigned to the A (1)Pi<-- X (1)Sigma(+) electronic transition of linear AlCCH. The vibronic band system is complicated by the Renner-Teller effect in the excited state. The assignment yields nu(4)' = 516.4 cm(-1) for the stretching mode in the ground X (1)Sigma(+) state and nu(4)' = 654.5 cm(-1) for A (1)Pi excited state. Molecular constants determined from the rotational analysis are B(0)' = 0.16487(14), B(0)' = 0.17845(13) and T(0) = 28 755.04 cm(-1). The experimental and theoretical data indicate a shorter Al-C bond in the A (1)Pi excited than the X (1)Sigma(+) ground state.     

Laser induced fluorescence spectroscopy of NC3O

Laser induced fluorescence spectra of the NC(3)O radical in a supersonic jet have been observed. The radical was produced in a pulsed electric discharge of HC(3)N and O(2) diluted to 0.3% with Ar. A total of 17 vibronic bands with a radiative lifetime of approximately 30 ns have been observed in a region from 27 000 to 27 500 cm(-1). The observed vibronic bands are classified as (2)Pi(12)-(2)Pi(12), (2)Pi(32)-(2)Pi(32), and (2)Sigma-(2)Sigma types. The upper states of the (2)Sigma-(2)Sigma bands have large spin-rotation constants, which should be denoted as Sigma((+)) and Sigma((-)). From high-level ab initio calculations and rotational analyses, the observed transition was assigned to the B (2)Pi-X (2)A(") transition. Dispersed fluorescence spectra from the upper (2)Sigma and (2)Pi vibronic levels have also been observed, yielding fundamental vibrational frequencies for the nu(1), nu(2), nu(3), and nu(7) modes of the ground state.     

The electronic spectrum of the C2P free radical and a Renner-Teller analysis of the 2Delta and X 2Pi electronic states

Subsequent to our spectroscopic detection of the C(2)X(X=P,As) free radicals [F. X. Sunahori et al., J. Am. Chem. Soc. 129, 9600 (2007)], we have studied the electronic spectrum of the (2)Delta(i)-X (2)Pi(r) system of the jet-cooled C(2)P free radical in the 490-630 nm region. The high-resolution laser-induced fluorescence spectrum of the two spin components of the 0(0) (0) band of (12)C(2)P has been recorded, and the rotational and spin-orbit coupling constants have been determined for both electronic states. The Renner-Teller effect has been observed in both the (2)Pi and the (2)Delta states, and the vibrational structure has been assigned. For the ground state, all of the observed levels up to 3500 cm(-1) were fitted with a standard Renner-Teller model. The excited (2)Delta state vibrational levels were successfully fitted using literature energy level expressions derived from perturbation theory, yielding vibrational and Renner-Teller parameters for both (12)C(2)P and (13)C(2)P. The molecular structure of C(2)P in the ground and excited states has also been estimated and compared to ab initio calculations and the geometries of similar molecules.