A microwave and quantum chemical study of cyclopropaneselenol

The microwave spectrum of cyclopropaneselenol, C 3H 5SeH, has been investigated in the 21.9-80 GHz frequency range. The microwave spectra of the ground vibrational state of five isotopologues of cyclopropaneselenol (C 3H 5 (82)SeH, C 3H 5 (80)SeH, C 3H 5 (78)SeH, C 3H 5 (77)SeH, and C 3H 5 (76)SeH) of one conformer, as well as the spectra of two vibrationally excited states of each of the C 3H 5 (80)SeH and C 3H 5 (78)SeH isotopologues of this rotamer, have been assigned. The H-C-Se-H chain of atoms is synclinal in this conformer, and there is no indication of further rotameric forms in the microwave spectrum. The b-type transitions of the ground vibrational state of the more abundant species C 3H 5 (80)SeH and C 3H 5 (78)SeH were split into two components, which is assumed to arise from tunneling of the proton of the selenol group between two equivalent synclinal potential wells. The tunneling frequencies were 0.693(55) MHz for C 3H 5 (80)SeH and 0.608(71) MHz for C 3H 5 (78)SeH. The microwave study has been augmented by high-level density functional and ab initio quantum chemical calculations, which indicate that the H-C-Se-H dihedral angle is approximately 75 degrees from synperiplanar (0 degrees).     

Microwave spectrum, conformation and intramolecular hydrogen bonding of 2,2,2-trifluoroethanethiol (CF3CH2SH)

The microwave spectrum of 2,2,2-trifluoroethanethiol, CF3CH2SH, and of one deuterated species, CF3CH2SD, has been investigated in the 7-80 GHz spectral interval. The microwave spectra of the ground and three vibrationally excited states belonging to three different normal modes of one conformer were assigned for the parent species, and the vibrational frequencies of these fundamentals were determined by relative intensity measurements. Only the ground vibrational state was assigned for the deuterated species. The identified form has a synclinal arrangement for the H-S-C-C chain of atoms and the corresponding dihedral angle is 68(5) degrees from synperiplanar (0 degrees). A weak intramolecular hydrogen bond formed between the thiol (SH) group and one of the fluorine atoms is stabilizing this conformer. There is no evidence in the microwave spectrum for the H-S-C-C antiperiplanar form. The hydrogen atom of the thiol group should have the ability to tunnel between two equivalent synclinal potential wells, but no splittings of spectral lines due to tunneling were observed. The microwave work was augmented by quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory.     

Synthesis, microwave spectrum, and dipole moment of allenylisocyanide (H2C═C═CHNC), a compound of potential astrochemical interest

An improved synthesis of a compound of potential astrochemical interest, allenylisocyanide (H(2)C═C═CHNC), is reported together with its microwave spectrum, which has been investigated in the 8-120 GHz spectral range to facilitate a potential identification in interstellar space. The spectra of the ground vibrational state and of five vibrationally excited states belonging to three different vibrational modes have been assigned for the parent species. A total of 658 transitions with a maximum value of J = 71 were assigned for the ground state and accurate values obtained for the rotational and quartic centrifugal distortion constants. The spectra of five heavy-atom ((13)C and (15)N) isotopologues were also assigned. The dipole moment was determined to be μ(a) = 11.93(16) × 10(-30) C m, μ(b) = 4.393(44) × 10(-30) C m, and μ(tot) = 12.71(16) × 10(-30) C m. The spectroscopic work has been augmented by theoretical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. The theoretical calculations are generally in good agreement with the experimental results.     

The microwave spectrum and structure of trifluoro-ethylene

The microwave spectrum of trifluoroethylene F₂C=CHF is reported, and a number of ground state and vibrationally excited state lines are assigned. The ground state rotational constants are: 10665.31, 3872.36, 2837.97 MHz. The dipole components are μ_a = 0.075 D, μ_b = 1.30 D, and μ_total = 1.30 D. Calculations of the inertia defect of the ground and excited states indicate that the equilibrium configuration is planar.     

高振动激发态吡嗪碰撞传能的QCT计算研究

用准经典轨线(QCT)方法计算了高振动激发态吡嗪(C4N2H4)与N2、O2、NH3、基态吡嗪之间的碰撞传能. C4N2H4通过计算发现, 高振动激发态C4N2H4与N2、O2碰撞发生的主要是V-V传能, 与NH3碰撞发生的主要是V-R传能, 与基态C4N2H4碰撞发生的主要是V-V(R)传能. 通过比较高振动激发态C4N2H4、C6F6、C6H6与其基态分子的碰撞传能， 发现此类碰撞传能中, 若分子的对称性高， 则V-V传能更容易实现.     

High-accuracy structure of cyclobutane by femtosecond rotational Raman four-wave mixing

The femtosecond degenerate four-wave mixing (fs-DFWM) technique is applied for the measurement of accurate rotational constants of cyclobutane (C4H8). The vibrational levels of C4H8 exhibit tunneling splitting due to the ring-puckering interconversion between the symmetry-equivalent D2d minima via a planar D4h barrier. For the v = 0 ground state, the fs-DFWM method yields a rotational constant B + 0 = 10663.452(18) MHz. The ring-puckering tunneling leads to slightly different rotational constants for the 0+ and 0- levels, B + 0 - B -0 = 33 +/- 2 kHz. This difference increases by a factor of approximately 90 in the v = 1+/1- ring-puckering states to B +1 - B -1 = -3059 +/- 4 kHz. Combining the experimental rotational constants with the structure parameters and rotational constants calculated by high-level ab initio calculations allows us to determine accurate equilibrium and vibrationally averaged structure parameters for cyclobutane, for example, re(C-C) = 1.5474 A, re(C-Haxial) = 1.0830 A, re(C-Hequatorial) = 1.0810 A, and ring puckering angle theta e = 29.8 degrees .     

Quasiclassical Trajectory Study of Collisional Energy Transfer between Highly Excited C6F6 and N2 ,O2 and Ground State C6F6

Quasiclassical trajectory calculation (QCT) is used frequently for studying collisional energy transfer between highly vibrationally excited molecules and bath gases. In this paper, the QCT of the energy transfer between highly vibrationally excited C6F6 and N2 ,O2 and ground state C6F6 were performed. The results indicate that highly vibrationally excited C6F6 transferred vibrational energy to vibrational distribution of N2, O2 and ground state C6F6, so they are V-V energy transfer. Especially it is mainly V-V resonance energy transfer between excited C6F6 andground state C6F6, excited C6F6 transfers more vibrational energy to ground state C6F6 than to N2 and O2. The values of QCT, - (ΔEvib) of excited C6F6 are smaller than those of experiments.     

丙烯酸分子的激发态超快预解离动力学

利用飞秒泵浦-探测技术结合飞行时间质谱(TOF-MS),研究了丙烯酸分子被200nm泵浦光激发到第二电子激发态(S2)后的超快预解离动力学.采集了母体离子和碎片离子的时间分辨质谱信号,并利用动力学方程对时间分辨离子质谱信号进行拟合和分析,揭示了预解离通道的存在.布居在S2激发态的分子通过快速的内转换弛豫到第一电子激发态(S1),时间常数为210fs,随后再经内转换从S1态弛豫到基态(S0)的高振动态,时间常数为1.49ps.分子最终在基态高振动态势能面上发生C-C键和C-O键的断裂,分别解离生成H2C=CH和HOCO、H2C=CHCO和OH中性碎片,对应的预解离时间常数分别约为4和3ps.碎片离子的产生有两个途径,分别来自于母体离子的解离和基态高振动态势能面上中性碎片的电离.     

A theoretical and experimental study on translational and internal energies of H2O and OH from the 157 nm irradiation of amorphous solid water at 90 K

The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.     

Microwave spectrum and conformational composition of 3-fluoropropionitrile (FCH2CH2CN)

The microwave spectrum of 3-fluoropropionitrile, FCH(2)CH(2)C≡N, has been investigated in the whole 17-75 GHz spectral region. Selected portions of the spectrum in the 75-95 GHz have also been recorded. The microwave spectra of the ground state as well as of three vibrationally excited states of each of two conformers have been assigned. The spectra of the vibrationally excited states belong to the lowest torsional and bending vibrations. The F-C-C-C chain of atoms is exactly antiperiplanar in one of these rotamers and synclinal in the second conformer. The F-C-C-C dihedral angle is 65(2)° in the synclinal form. The energy difference between the two forms has been obtained from relative intensity measurements performed on microwave transitions. It was found that the antiperiplanar conformer is more stable than the synclinal form by 1.4(5) kJ/mol. It is argued that the gauche effect is a significant force in this compound. Quantum chemical calculations at the high CCSD(full)/cc-pVTZ, MP2(full)/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the theoretical predictions are in good agreement with experiment.     

Quantum-dynamics study of the H5+ cluster: full dimensional benchmark results on its vibrational states

A full-dimensional quantum dynamics study is carried out for the highly fluxional H(5)(+) cation on a recent reference potential energy surface by using the multi configuration time-dependent Hartree method. With five equivalent light atoms and shallow barriers between various low-lying stationary points on the surface, the spectroscopic characterization of H(5)(+) represents a huge challenge for accurate quantum dynamics simulations. The present calculation is the first such a study on this cation, which together with its isotope analogies are of primary importance in the interstellar chemistry. The vibrational ground state properties and several vibrationally excited states corresponding to low vibrational frequency motions, not yet directly observable by the experiment, are presented and analyzed.     

Iterative diagonalization in the state-averaged multi-configurational time-dependent Hartree approach: excited state tunneling splittings in malonaldehyde

An iterative block Lanczos-type diagonalization scheme utilizing the state-averaged multi-configurational time-dependent Hartree (MCTDH) approach is introduced. Combining propagation in real and imaginary time and using a set of initial seed wavefunctions corresponding to excitations via the different components of the dipole moment vector, the scheme can favorably be used to selectively compute vibrational states which show high intensities in vibrational absorption spectra. Tunneling splitted vibrational states in double well systems can be described particularly efficient employing an increased set of seed wavefunctions which includes symmetric and anti-symmetric wavefunctions simultaneously. The new approach is used to study the tunneling splittings of the vibrationally excited states of malonaldehyde. Full-dimensional multi-layer MCTDH calculations are performed and results for the tunneling splittings of several excited vibrational states can be obtained. The calculated tunneling splittings agree reasonably well with available experimental data. Order of magnitude differences between tunneling splittings of different vibrationally excited states are found and interpreted.     

Quantum dynamics of the Li + HF --> H + LiF reaction at ultralow temperatures

Quantum-mechanical calculations are reported for the Li+HF(v=0,1,j=0)-->H+LiF(v',j') bimolecular scattering process at low and ultralow temperatures. Calculations have been performed for zero total angular momentum using a recent high-accuracy potential-energy surface for the X2A' electronic ground state. For Li+HF(v=0,j=0), the reaction is dominated by resonances due to the decay of metastable states of the Li cdots,...F-H van der Waals complex. Assignment of these resonances has been carried out by calculating the eigenenergies of the quasibound states. We also find that while chemical reactivity is greatly enhanced by vibrational excitation, the resonances get mostly washed out in the reaction of vibrationally excited HF with Li atoms. In addition, we find that at low energies, the reaction is significantly suppressed due to the less-efficient tunneling of the relatively heavy fluorine atom.     

Unraveling the spectroscopy of coupled intramolecular tunneling modes: a study of double proton transfer in the formic-acetic acid complex

The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states. The transitions are fitted to within a few kilohertz of the observed frequencies using a molecule-fixed effective rotational Hamiltonian for the separate A and E vibrational species of the G(12) permutation-inversion symmetry group. Interpretation of the motion problem uses an internal-vibration and overall-rotation angular momentum coupling scheme and full sets of rotational and centrifugal distortion constants are determined. The tunneling frequencies of the proton-transfer motion are measured for the ground A and E methyl rotation states as 250.4442(12) and -136.1673(30) MHz, respectively. The slight deviation of the latter tunneling frequency from being one half of the former, as simple theory otherwise predicts, is due to different degrees of mixing in wavefunctions between the ground and excited states.     

Intermolecular vibrations and dynamics of the anisole—benzene complex studied by multi-resonance spectroscopy

The intermolecular vibrations of the anisole—benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27 cm⁻¹, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 μs. On the other hand, the complex excited to the intramolecular vibrational states above 500 cm⁻¹ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.     

Reaction rates for O + HD → OH + D and O + HD → OD + H

We have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least-action ground-state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H₂ → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited-vibrational state reactions for T > 600 K. For the excited-state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited-state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.     

Near-ultraviolet photodissociation of thiophenol

H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the dynamics of H(D) atom loss C6H5SH(C6H5SD) following excitation at many wavelengths lambda phot in the range of 225-290 nm. The C6H5S cofragments are formed in both their ground (X(2)B1) and first excited ((2)B2) electronic states, in a distribution of vibrational levels that spreads and shifts to higher internal energies as lambda(phot) is reduced. Excitation at lambda(phot) > 275 nm populates levels of the first (1)pi pi* state, which decay by tunnelling to the dissociative (1)pi sigma* state potential energy surface (PES). S-H torsional motion is identified as a coupling mode facilitating population transfer at the conical intersection (CI) between the diabatic (1)pi pi* and (1)pi sigma* PESs. At shorter lambda(phot), the (1)pi sigma* state is deduced to be populated either directly or by efficient vibronic coupling from higher (1)pipi* states. Flux evolving on the (1)pi sigma* PES samples a second CI, at longer R(S-H), between the diabatic (1)pi sigma* and ground ((1)pi pi) PESs, where the electronic branching between ground and excited state C6H5S fragments is determined. The C6H5S(X(2)B1) and C6H5S((2)B2) products are deduced to be formed in levels with, respectively, a' and a' vibrational symmetry-behavior that reflects both Franck-Condon effects (both in the initial photoexcitation step and in the subsequent in-plane forces acting during dissociation) and the effects of the out-of-plane coupling mode(s), nu11 and nu16a, at the (1)pi sigma*/(1)pi pi CI. The vibrational state assignments enabled by the high-energy resolution of the present data allow new and improved estimations of the bond dissociation energies, D0(C6H5S-H) < or = 28,030 +/- 100 cm(-1) and D0(C6H5S-D) < or = 28,610 +/- 100 cm(-1), and of the energy separation between the X(2)B1 and (2)B2 states of the C6H5S radical, T(00) = 2800 +/- 40 cm(-1). Similarities, and differences, between the measured energy disposals accompanying UV photoinduced X-H (X = S, O) bond fission in thiophenol and phenol are discussed.     

Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

The SH + CH(3) product channel for the photodissociation of CH(3)SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH(3) products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH(3) formed in the ground and excited vibrational states (v(2) = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH(3) channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH(3) (v(2) = 0, 1, and 2) products is indicative of a fast dissociation process for C-S bond cleavage. Two kinds of slower CH(3) products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH(3)S (X(2)E) radical via cleavage of the S-H bond in CH(3)SH, followed by probe laser photodissociation of nascent CH(3)S radicals yielding CH(3)(X(2)A(1), v(2) = 0-2) + S((3)P(j)/(1)D) products.     

Imaging studies of the photodissociation of H2S+ cations. II

Ion imaging methods have been used to explore the photodissociation dynamics of state-selected H(2)S(+) and D(2)S(+) cations. Predissociation following one photon excitation to the A (2)A(1) state at wavelengths (385< or =lambda(phot)< or =420 nm) in the vicinity of the first dissociation threshold results in formation of ground state S(+) fragment ions; the partner H(2)(D(2)) fragments are deduced to be rotationally "cold." Two photon dissociation processes are also observed, resonance enhanced at the energy of one absorbed photon by the predissociating A state levels. Two photon excitation at these wavelengths is deduced to populate an excited state of (2)A(1) symmetry, which dissociates to electronically excited S(+)((2)D) products, together with vibrationally excited H(2)(D(2)) cofragments. Ground state SH(+)(SD(+)) fragments, attributable to a one photon dissociation process, are observed once lambda(phot)< or =325 nm. Two photon induced production of SH(+)(SD(+)) fragments is also observed, at all wavelengths studied (i.e., at all lambda(phot)< or =420 nm). These SH(+)(SD(+)) fragments are deduced to be formed in their singlet (i.e., a (1)Delta and b (1)Sigma(+)) excited states, with high levels of rotational excitation. The observed product branching and energy disposals are discussed within the context of the (limited) available knowledge relating to the excited electronic states of the H(2)S(+) cation.     

Vibrationally mediated photodissociation of ammonia: the influence of N-H stretching vibrations on passage through conical intersections

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N-H stretch produces primarily ground state NH(2) along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N-H stretch. Decomposition from that state produces almost solely electronically excited NH(2)*, avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH(2)* suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.