SiH2Cl2: ab initio anharmonic force field, dipole moments, and infrared vibrational transitions

The vibrational spectra of SiH2Cl2 have been recorded in the 1000-13,000 cm(-1) region, utilizing the Fourier-transform spectroscopy and Fourier-transform intracavity laser absorption spectroscopy. Totally 61 band centers and intensities are derived from the infrared spectra. An ab initio quartic force field is obtained by applying the second-order Moller-Plesset perturbation theory and correlation-consistent polarized valence triplet-zeta basis sets [J. Chem. Phys. 90, 1007 (1989); 98, 1358 (1993)]. Most observed bands are assigned by the vibration analysis based on the second-order perturbation theory. Reduced-dimensional ab initio dipole moment functions (two dimensional and three dimensional) have also been calculated to investigate the absolute band intensities of the SiH2 chromophore. The calculated values agree reasonably with the observed ones.     

Ab initio and RRKM study of the HCN/HNC elimination channels from vinyl cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).     

Dipole moment and rovibrational intensities in the electronic ground state of NH3: bridging the gap between ab initio theory and spectroscopic experiment

We report theoretical values for the transition moments of an extensive set of vibrational bands in the electronic ground state of (14)NH(3). For selected bands, we have further made detailed simulations of the rotational structure. The calculations are carried out by means of recently developed computational procedures for describing the nuclear motion and are based on a high-level ab initio potential energy surface, and high-level dipole moment surfaces, for the electronic ground state of NH(3). The reported theoretical intensity values are compared to, and found to agree very well with, corresponding experimental results. It is believed that the computational method, in conjunction with high-quality ab initio potential energy and dipole moment surfaces, can simulate rotation-vibration spectra of XY(3) pyramidal molecules prior to observation with sufficient accuracy to facilitate the observation of these spectra. By degrading the accuracy of selected elements of the calculations, we have also investigated the influence of customary approximations on the computed intensity values.     

In search of the germanium halomethylidyne (Ge=C-X; X=F,Cl,Br) free radicals: Ab initio studies of their spectroscopic signatures

A variety of ab initio methods have been used to calculate the X (2)Pi and A (2)Sigma(+) state spectroscopic parameters of the GeCX (X=F,Cl,Br) free radicals. The theoretical methods and basis sets were tested on GeCH, for which extensive experimental data are available, and found to give predictions sufficiently reliable to guide experimental searches for spectra. In all cases, the linear Ge=C-X species was found to be the global minimum on the potential energy surface, with the bent X-Ge=C ((2)A(')) isomer as a local minimum much higher (62-36 kcal/mol) in energy. In both the ground and excited states, the GeC moiety is very similar to that of GeCH, with a double bond in the lower state and a triple bond in the excited state, indicating that halogenation does not radically perturb the energetics or structure of germanium methylidyne. Ground state GeCX radicals have suitable rotational constants for microwave studies, although they suffer from only modest dipole moments. Matrix infrared experiments are most likely to detect the nu(1) fundamentals in the 1450-1100 cm(-1) region or the nu(3) fundamentals at the transition between the mid- and far-infrared regions. We have used the ab initio values for the Renner-Teller parameter, the average bending frequency, and the spin-orbit coupling constant to calculate the ground state energy levels, which will be helpful in the interpretation of A-X single vibronic level emission spectra, if they can be observed. The electronic absorption spectra of the (2)Pi(32) spin component of the 0(0) (0) bands of all three radicals have been calculated assuming typical jet-expansion conditions and should be useful in future laser-induced fluorescence, resonance enhanced multiphoton ionization, or cavity ringdown searches for the electronic band systems.     

Theoretical study of HCN and HNC neutral and charged clusters

A theoretical study of linear and cyclic clusters of (HCN)n and (HNC)n (up to n = 10) has been carried out by means of DFT and MP2 ab initio methods. The transition states linking the cyclic clusters show high energetic barriers that prevent the spontaneous transformation of the high-energy clusters, (HNC)n, into the low-energy ones, (HCN)n. The effect of the protonation/deprotonation of the linear clusters has also been explored. The results show that (HNC)n clusters with n values larger than six are thermodynamically more stable as charged systems than as neutral ones. The geometrical results have been analyzed using a Steiner-Limbach plot. The electron density and its Laplacian at the bond critical points correlate with the corresponding bond distances by means of two exponential functions, one for the open shell and another for the closed shell cases.     

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: a new ab initio dipole surface for high-temperature applications

We report new ab initio results for the interaction-induced dipole moments Δμ of hydrogen molecules colliding with helium atoms. These results are needed in order to calculate collision-induced absorption spectra at high temperatures; applications include modeling the radiative profiles of very cool white dwarf stars, with temperatures from 3500 K to 9000 K. We have evaluated the dipoles based on finite-field calculations, with coupled cluster methods in MOLPRO 2006 and aug-cc-pV5Z (spdfg) basis sets for both the H and He centers. We have obtained values of Δμ for eight H(2) bond lengths ranging from 0.942 a.u. to 2.801 a.u., for 15 intermolecular separations R ranging from 2.0 a.u. to 10.0 a.u., and for 19 different relative orientations. In general, our values agree well with earlier ab initio results, for the geometrical configurations that are treated in common, but we have determined more points on the collision-induced dipole surface by an order of magnitude. These results make it possible to calculate transition probabilities for molecules in excited vibrational states, overtones, and rotational transitions with ΔJ > 4. We have cast our results in the symmetry-adapted form needed for absorption line shape calculations, by expressing Δμ as a series in the spherical harmonics of the orientation angles of the intermolecular vector and of a unit vector along the H(2) bond axis. The expansion coefficients depend on the H(2) bond length and the intermolecular distance R. For large separations R, we show that the ab initio values of the leading coefficients converge to the predictions from perturbation theory, including both classical multipole polarization and dispersion effects.     

An ab initio study of the structures and the harmonic and anharmonic stretching force constants of the cyanides HCN,LiCN, FCN,ClCN and isocyanides HNC,LiNC, FNC,ClNC

Equilibrium structures and both harmonic and anharmonic stretching force constants have been calculated ab initio for the four cyanides HCN, LiCN, FCN, and C1CN, and the four isocyanides HNC, LiNC, FNC, and ClNC, using several basis sets of about triple-zeta quality. The CN and NC bond lengths, as well as diagonal stretching force constants, are nearly the same throughout both series. Trends in the off-diagonal stretching force constants are discussed, and dipole moment values are correlated with the structural type, XCN and XNC, and the electronegativity of the ligand X.     

Jet cooled spectroscopy of H2DO+: Barrier heights and isotope-dependent tunneling dynamics from H3O+ to D3O+

The first high resolution spectroscopic data for jet cooled H2DO+ are reported, specifically via infrared laser direct absorption in the OH stretching region with a slit supersonic jet discharge source. Transitions sampling upper (0-) and lower (0+) tunneling states for both symmetric (nu1+ <-- 0+, nu1- <-- 0-, and nu1- <-- 0+) and antisymmetric (nu3+ <-- 0+ and nu3- <-- 0-) OH stretching bands are observed, where +/- refers to wave function reflection symmetry with respect to the planar umbrella mode transition state. The spectra can be well fitted to a Watson asymmetric top Hamiltonian, revealing band origins and rotational constants for benchmark comparison with high-level ab initio theory. Of particular importance are detection and assignment of the relatively weak band (nu1- <-- 0+) that crosses the inversion tunneling gap, which is optically forbidden in H3O+ or D3O+, but weakly allowed in H2DO+ by lowering of the tunneling transition state symmetry from D(3h) to C(2v). In conjunction with other H2DO+ bands, this permits determination of the tunneling splittings to within spectroscopic precision for each of the ground [40.518(10) cm(-1)], nu1 = 1 [32.666(6) cm(-1)], and nu3 = 1 [25.399(11) cm(-1)] states. A one-dimensional zero-point energy corrected potential along the tunneling coordinate is constructed from high-level ab initio CCSD(T) calculations (AVnZ, n = 3,4,5) and extrapolated to the complete basis set limit to extract tunneling splittings via a vibrationally adiabatic treatment. Perturbative scaling of the potential to match splittings for all four isotopomers permits an experimental estimate of DeltaV0 = 652.9(6) cm(-1) for the tunneling barrier, in good agreement with full six-dimensional ab initio results of Rajamaki, Miani, and Halonen (RMH) [J. Chem. Phys. 118, 10929 (2003)]. (DeltaV0 (RMH) = 650 cm(-1)). The 30%-50% decrease in tunneling splitting observed upon nu1 and nu3 vibrational excitations arises from an increase in OH stretch frequencies at the planar transition state, highlighting the transition between sp2 and sp3 hybridizations of the OHD bonds as a function of inversion bending angle.     

HNC/HCN ratio in acetonitrile, formamide, and BrCN discharge

Time-resolved Fourier transform (FT) spectrometry was used to study the dynamics of radical reactions forming the HCN and HNC isomers in pulsed glow discharges through vapors of BrCN, acetonitrile (CH(3)CN), and formamide (HCONH(2)). Stable gaseous products of discharge chemistry were analyzed by selected ion flow tube mass spectrometry (SIFT-MS). Ratios of concentrations of the HNC/HCN isomers obtained using known transition dipole moments of rovibrational cold bands v(1) were found to be in the range 2.2-3%. A kinetic model was used to assess the roles the radical chemistry and ion chemistry play in the formation of these two isomers. Exclusion of the radical reactions from the model resulted in a value of the HNC/HCN ratio 2 orders of magnitude lower than the experimental results, thus confirming their dominant role. The major process responsible for the formation of the HNC isomer is the reaction of the HCN isomer with the H atoms. The rate constant determined using the kinetic model from the present data for this reaction is 1.13 (±0.2) × 10(-13) cm(3) s(-1).     

Measurement of the electronic transition dipole moment by Autler-Townes splitting: Comparison of three- and four-level excitation schemes for the Na2 A 1Sigma(u)+ - X 1Sigma(g)+ system

We present a fundamentally new approach for measuring the transition dipole moment of molecular transitions, which combines the benefits of quantum interference effects, such as the Autler-Townes splitting, with the familiar R-centroid approximation. This method is superior to other experimental methods for determining the absolute value of the R-dependent electronic transition dipole moment function mu(e)(R), since it requires only an accurate measurement of the coupling laser electric field amplitude and the determination of the Rabi frequency from an Autler-Townes split fluorescence spectral line. We illustrate this method by measuring the transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) rovibronic transition and compare our experimental results with our ab initio calculations. We have compared the three-level (cascade) and four-level (extended Lambda) excitation schemes and found that the latter is preferable in this case for two reasons. First, this excitation scheme takes advantage of the fact that the coupling field lower level is outside the thermal population range. As a result vibrational levels with larger wave function amplitudes at the outer turning point of vibration lead to larger transition dipole moment matrix elements and Rabi frequencies than those accessible from the equilibrium internuclear distance of the thermal population distribution. Second, the coupling laser can be "tuned" to different rovibronic transitions in order to determine the internuclear distance dependence of the electronic transition dipole moment function in the region of the R-centroid of each coupling laser transition. Thus the internuclear distance dependence of the transition moment function mu(e)(R) can be determined at several very different values of the R centroid. The measured transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) transition is 5.5+/-0.2 D compared to our ab initio value of 5.9 D. By using the R-centroid approximation for this transition the corresponding experimental electronic transition dipole moment is 9.72 D at Rc = 4.81 A, in good agreement with our ab initio value of 10.55 D.     

Water line lists close to experimental accuracy using a spectroscopically determined potential energy surface for H2(16)O, H2(17)O, and H2(18)O

Line lists of vibration-rotation transitions for the H(2) (16)O, H(2) (17)O, and H(2) (18)O isotopologues of the water molecule are calculated, which cover the frequency region of 0-20 000 cm(-1) and with rotational states up to J=20 (J=30 for H(2) (16)O). These variational calculations are based on a new semitheoretical potential energy surface obtained by morphing a high accuracy ab initio potential using experimental energy levels. This potential reproduces the energy levels with J=0, 2, and 5 used in the fit with a standard deviation of 0.025 cm(-1). Linestrengths are obtained using an ab initio dipole moment surface. That these line lists make an excellent starting point for spectroscopic modeling and analysis of rotation-vibration spectra is demonstrated by comparison with recent measurements of Lisak and Hodges [J. Mol. Spectrosc. (unpublished)]: assignments are given for the seven unassigned transitions and the intensity of the strong lines are reproduced to with 3%. It is suggested that the present procedure may be a better route to reliable line intensities than laboratory measurements.     

H2 + CN(n=0,1)→H + HCN振动选态反应

用从头算方法获得了Ｈ２＋ＣＮ反应的内禀反应坐标（ＩＲＣ），沿着ＩＲＣ，计算了各垂直于ＩＲＣ的简正模所对应的频率（Ｗ）以及沿ＩＲＣ运动与垂直ＩＲＣ运动的简正模之间的耦合常数（ＢＫＦ），根据传统过渡态，变分过渡态理论和选态公式，计算了ｎＣＮ＝０及ｎＣＮ＝１时反应的速率常数，并得到了实验相一致的结果，还计算了ｎＣＨ＝１及ｎＣＮ＝１的Ｈ＋ＨＣＮ→Ｈ２＋ＣＮ反应速率常数，可供实验工作者参考。     

Comment on "Theory of photodissociation of ozone in the Hartley continuum; effect of vibrational excitation and O(1D) atom velocity distribution" by E. Baloïtcha and G. G. Balint-Kurti, Phys. Chem. Chem. Phys., 2005, 7, 3829

In this Comment we present quantum mechanical absorption spectra of the Hartley band originating from the four vibrationally excited levels of the ground electronic state. The calculations are performed using the diabatic B-state potential energy surface and the transition dipole moment vector constructed from the ab initio data of the title paper. The calculated spectra are multimodal (for the symmetric stretch pre-excitation) and strongly structured (for the symmetric stretch and bending pre-excitations). These results agree with the previous theoretical analysis and with the predictions of a simple model based on the reflection principle, but contradict the findings of Balo?tcha amd Balint-Kurti thus questioning the accuracy of their calculations.     

Bound states and scattering resonances of OH(A)-He

The OH-He complex has been observed using laser excitation of the A 2sigma+-X 2pi transition. The bands of the complex were close to the monomer rotational lines that terminate on the n = 0, 1, and 2 levels of OH(A). The unresolved band associated with He.OH (A,n=0) was redshifted from the OH parent line by 1.6 cm(-1), providing a direct measurement of D0'-D0". The complex features associated with n = 1 and 2 were identified as scattering resonances. They have been assigned by comparison with resonance structures derived from close-coupling calculations. The ab initio potential energy surface of H.-S. Lee, A.B. McCoy, R.R. Toczylowski, and S.M. Cybulski, [J. Chem. Phys. 113, 5736 (2000)] was used in these calculations. The level of agreement between the observed and predicted resonances indicated that the ab initio surface is reasonably accurate.     

Energy-conformation studies of HCN,HNC and CN−: A comparison of results from EH-SCC and SCF molecular orbital calculations

We have made an Extended Hückel Self Consistent Charge (EH-SCC) molecular orbital calculation for hydrogen cyanide, hydrogen isocyanide and cyanide ion. The main purpose of this calculation was to compare the EH-SCC and the more accurate SCF MO calculations for HCN in order to evaluate the method we used here for future use. Specifically, we have calculated and compared the following properties of HCN: total energy, binding energy, variation of ground state energy with geometric conformation, ionization potential and dipole moment. In addition, we have extended previous calculations of HCN by also considering its energy variation with bond angle for two excited state configurations and deducing some of the characteristics of its electronic spectra. Finally we have also made an MO calculation of the isocyanide isomer HNC and CN^– ion to compare with and add to the known characterization of the H, C, N, system.

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Zusammenfassung Rechnungen nach der erweiterten Hückeltheorie werden für HCN, HNC und CN^– durchgeführt und mit ab initio Resultaten verglichen. Im einzelnen wurden Gesamtenergie, Bindungsenergie in Abhängigkeit von der geometrischen Struktur, Ionisierungspotential und Dipolmoment von HCN berechnet und außerdem die Energie für zwei doppelt angeregte Konfigurationen in Abhängigkeit vom Bindungswinkel bestimmt. Darüber hinaus sind MO-Rechnungen für HNC und CN^– gemacht worden.

     

Infrared spectroscopy of HCN-salt complexes formed in liquid-helium nanodroplets

Rotationally resolved infrared spectra are reported for the binary complexes of HCN and LiF, LiCl, NaF, and NaCl, formed in helium nanodroplets. Stark spectroscopy is used to determine the dipole moments for these complexes. Ab initio calculations are also reported for these complexes, revealing the existence of several different isomers of these binary systems. In the frequency region examined in this experimental study we only observe one of these, corresponding to the salt binding to the nitrogen end of the HCN molecule. The experimental rotational constants, dipole moments, and vibrational frequency shifts are all compared with the results from ab initio calculations for this isomer.     

IR spectra of N-methylacetamide in water predicted by combined quantum mechanical/molecular mechanical molecular dynamics simulations

We applied the combined quantum mechanical (QM)/molecular mechanical (MM) molecular dynamics (MD) simulation method in assessing IR spectra of N-methylacetamide and its deuterated form in aqueous solutions. The model peptide is treated at the Austin Model 1 (AM1) level and the induced dipole effects by the solvent are incorporated in fluctuating solute dipole moments, which are calculated using partial charges from Mulliken population analyses without resorting to any available high-level ab initio dipole moment data. Fourier transform of the solute dipole autocorrelation function produces in silico IR spectra, in which the relative peak intensities and bandwidths of major amide bands are quantitatively compatible with experimental results only when both geometric and electronic polarizations of the peptide by the solvent are dealt with at the same quantum-mechanical level. We cast light on the importance of addressing dynamic charge fluctuations of the solute in calculating IR spectra by comparing classical and QM/MM MD simulation results. We propose the adjustable scaling factors for each amide mode to be directly compared with experimental data.     

Theoretical investigation of ground and excited states of the methylene amidogene radical (H(2)CN)

The excited states and the absorption spectrum of the methylene amidogene radical are studied by high-level ab initio calculations. The multireference configuration interaction method was used in combination with different basis sets and basis set extrapolation to compute equilibrium geometries, harmonic frequencies, and excitation energies of the four lowest doublet electronic states of the title species. Potential curves and transition dipole moment functions were determined along the normal mode coordinates of the electronic ground state. These functions were employed to determine vibronic absorption spectra. The intensities of dipole forbidden but vibronically allowed transitions were calculated by explicitly evaluating integrals over the vibrational wave functions and the transition dipole functions of the involved electronic states. By this method the oscillator strengths of the dipole allowed (2)A(1)<--(2)B(2) and the dipole forbidden (2)B(1)<--(2)B(2) bands were computed. It turns out that the dipole forbidden transition is two orders of magnitude weaker than the dipole allowed one. The 0-0 excitation energies are found to be 30 256 cm(-1) for the (2)B(1) state and 34,646 cm(-1) for the (2)A(1) state. From the combined results of the excitation energies and oscillator strengths it is concluded that the experimentally observed peaks must be due to the (2)A(1) state, in contradiction to earlier assignments.     

Electronic spectroscopy of the A1A' <-- X1A' system of CDF

To further investigate the Renner-Teller (RT) effect and barriers to linearity and dissociation in the simplest singlet carbene, we recorded fluorescence excitation spectra of bands involving the pure bending levels 2(n)(0) with n = 0-9 and the combination states 1(1)(0)2(n)(0) with n = 1-8 and 2(n)(0)3(1)(0) with n = 0-5 in the A(1)A'<-- X(1)A' system of CDF, in addition to some weak hot bands. The spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants; fluorescence lifetimes were also measured to probe for lifetime lengthening effects due to the RT interaction. The derived A state parameters are compared with previous results for CHF and with predictions of ab initio electronic structure theory. The approach to linearity in the A state is evidenced in a sharp increase in the A rotational constant with bending excitation, and a minimum in the vibrational intervals near 2(9). A fit of the vibrational intervals for the pure bending levels yields an A state barrier to linearity in good agreement both with that previously derived for CHF and ab initio predictions. From the spectra and lifetime measurements, the onset of extensive RT perturbations is found to occur at a higher energy than in CHF, consistent with the smaller A constant.     

Fluorescence and REMPI spectroscopy of jet-cooled isolated 2-phenylindene in the S1 state

We investigated the spectroscopy of the first excited singlet electronic state S1 of 2-phenylindene using both fluorescence excitation spectroscopy and resonantly enhanced multiphoton ionization spectroscopy. Moreover, we investigated the dynamics of the S1 state by determining state-selective fluorescence lifetimes up to an excess energy of approximately 3400 cm(-1). Ab initio calculations were performed on the torsional potential energy curve and the equilibrium and transition state geometries and normal-mode frequencies of the first excited singlet state S1 on the CIS level of theory. Numerous vibronic transitions were assigned, especially those involving the torsional normal mode. The torsional potentials of the ground and first excited electronic states were simulated by matching the observed and calculated torsional frequency spacings in a least-squares fitting procedure. The simulated S1 potential showed very good agreement with the ab initio potential calculated on the CIS/6-31G(d,p) level of theory. TDDFT energy corrections improved the match with the simulated S(1) torsional potential. The latter calculation yielded a torsional barrier of V2 = 6708 cm(-1), and the simulation a barrier of V2 = 6245 cm(-1). Ground-state normal-mode frequencies were calculated on the B3LYP/6-31G(d,p) level of theory, which were used to interpret the infrared spectrum, the FDS spectrum of the transition and hot bands of the FES spectrum. The fluorescence intensities of the nu49 overtone progression could reasonably be reproduced by considering the geometry changes upon electronic excitation predicted by the ab initio calculations. On the basis of the torsional potential calculations, it could be ruled out that the uniform excess energy dependence of the fluorescence lifetimes is linked to the torsional barrier in the excited state. The rotational band contour simulation of the transition yielded rotational constants in close agreement to the ab initio values for both electronic states. Rotational coherence signals were obtained by polarization-analyzed, time-resolved measurements of the fluorescence decay of the transition. The simulation of these signals yielded corroborating evidence as to the quality of the ab initio calculated rotational constants of both states. The origin of the anomalous intensity discrepancy between the fluorescence excitation spectrum and the REMPI spectrum is discussed.