Intensity Measurements of Deltal > 1 Transitions of Several Isotopomers of HCN

Size-Dependent πg + πu Combination Band Intensities of Polyynes C2nH2 (n = 1-9) Analyzed by the Local CCH Bending and the Linear Response Functions

Polyynes (C_2nH₂) show the unusually strong π_g + π_u combination bands in the infrared absorption spectra. We calculated them as the first overtone of the local CCH bending; the strong intensities are interpreted as a consequence of the large-amplitude bending vibration of the acidic acetylenic hydrogen combined with the size-dependent π electron conjugation. Our theoretical calculations show that the absorption intensity increases steadily and their increase rate is gradually slowed down by increasing the number of acetylene units up to n = 9. However, the calculated vibrational wavenumber converges quickly in agreement with the experimental observation. The second-order electron density deformation caused by the local CCH bending was analyzed using the linear response functions, including the linear and nonlinear contributions, to explain the n dependence. The easily polarizable π electron density caused two kinds of deformation-dominant but dark δ_xx-yy type and minor but bright σ type. Both of them exhibit interesting zigzag sign alternations, consistent with the law of alternating polarity of Coulson and Longuet-Higgins. The electron density polarization in these intra- and interacetylene units induces a large axial component molecular dipole moment, contributing to the intensity that increases with n. The difference between the curvilinear and rectilinear bending coordinates is interpreted within the present theoretical scheme.

High Accuracy ab Initio Calculations of Rotational-Vibrational Levels of the HCN/HNC System

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born-Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational-vibrational energy levels up to 25 000 cm^-1 which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm^-1. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm^-1 for four HCN isotopologues (HCN, DCN, H¹³CN, and HC¹⁵N) being reproduced with σ = 0.37 cm^-1. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm^-1, respectively.

New Double Many-Body Expansion Potential Energy Surface for Ground-State HCN from a Multiproperty Fit to Accurate ab Initio Energies and Rovibrational Calculations

An accurate single-sheeted double many-body expansion potential energy surface has been obtained for the ground electronic state of the hydrogen cyanide molecule via a multiproperty fit to ab initio energies and rovibrational data. This includes 106 rovibrational levels and 2313 discrete points, which are fit with a rmsd of 4 cm(-1) and 2.42 kcal mol(-1), respectively, and seven zero first-derivatives that are reproduced at three stationary points. Since the potential also describes accurately the appropriate asymptotic limits at the various dissociation channels, it is commended both for the simulation of rovibrational spectra and reaction dynamics.

Spectroscopy of highly excited vibrational states of HCN in its ground electronic state

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29,000 and 30,000 cm(-1). The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the nu3 mode being approximately equivalent to the C-H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A 2Pi electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A-->B-->X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation.

Ab initio rotation-vibration spectra of HCN and HNC

We have calculated an ab initio HCN/HNC linelist for all transitions up to J= 25 and 18000 cm(-1) above the zero point energy. This linelist contains more than 200 million lines each with frequencies and transition dipoles. The linelist has been calculated using our semi-global HCN/HNC VQZANO + PES and dipole moment surface, which were reported in van Mourik et al. (J. Chem. Phys. 115 (2001) 3706). With this linelist we synthesise absorption spectra of HCN and HNC at 298 K and we present the band centre and band transition dipoles for the bands which are major features in these spectra. Several of the HCN bands and many of the HNC bands have not been previously studied. Our line intensities reproduce via fully ab initio methods the unusual intensity structure of the HCN CN stretch fundamental (00(0)1) for the first time and also the forbidden (02(2)0) HCN bending overtone. We also compare the J = 1-->0 pure rotational transition dipole in the HCN/HNC ground and vibrationally excited states with experimental and existing ab initio results.

Absolute Intensities Measurements in the nu(4) + nu(5) Band of (12)C(2)H(2): Analysis of Herman-Wallis Effects and Forbidden Transitions

We measured absolute line intensities in two bands of (12)C(2)H(2) near 7.5 µm, namely the nu(4) + nu(5)(Sigma(+)(u))-0(Sigma(+)(g)) and nu(4) + nu(5)(Delta(u))-0(Sigma(+)(g)) bands, using Fourier transform spectroscopy with an accuracy estimated to be better than 2%. Using theoretical predictions from Watson [J. K. G. Watson, J. Mol. Spectrosc. 188, 78 (1998)], the observation of the forbidden nu(4) + nu(5)(Delta(u))-0(Sigma(+)(g)) band and the Herman-Wallis behavior exhibited by its rotational lines were studied quantitatively in terms of two types of interactions affecting the levels involved by the band: l-type resonance and Coriolis interaction. In the case of the nu(4) + nu(5)(Sigma(+)(u))-0(Sigma(+)(g)) band, the influence of l-type resonance is also confirmed. We also attributed the intensity asymmetry observed between the R and P branches of that latter band to a Coriolis interaction with l = 1 levels. We did not observe the nu(4) + nu(5)(Sigma(-)(u))-0(Sigma(+)(g)) band, consisting only of a Q branch, in agreement with Watson's prediction. Copyright 2000 Academic Press.

Emission spectra of HCN/HNC in the 2-5 microm range of astrophysical interest

Emission from H12C14N observed with a Fourier transform spectrometer from a radio-frequency excited plasma is reported in the 2400-3400 cm(-1) spectral range of astrophysical interest. The molecular constants, for 21 vibration-rotation bands are given, as well as estimates of the first-order Herman-Wallis coefficients for 11 bands. These constants are derived from about 900 observed transitions in HCN, and are used to generate a sequential linelist of about 1400 calculated line positions, within a standard deviation equal to 3 x 10(-4) cm(-1). The relative intensities of the observed lines are also reported, as well as those for the nu1 band of H14N12C, at 3650 cm(-1), simultaneously observed from the same plasma.

Climbing the Bending Vibrational Ladder in D13C15N by Hot Gas Emission Spectroscopy

Using a newly constructed Fourier transform emission apparatus, we have measured the threefold substituted HCN isotopomer, D13C15N, at 1370 K in the range from 450 to 700 cm-1. We could assign hot bands with upper states up to Kvl2 = 12(12). The assignments have been verified for states up to v2 = 5 by fitting with earlier room temperature absorption measurements of overtone and hot bands. The intensities are shown to be in qualitative agreement with the expected intensity pattern for such emission spectra. All the measurements for D13C15N have been combined in a single least-squares fit that includes approximately 2700 rovibrational lines which have a root-mean-square deviation on the order of 0.000 4 cm-1. The spectroscopic constants for the bending states v2 = 1, ellipsis,12 are reported, as well as for some combination states involving the other vibrational modes. We also give the spectroscopic constants of various states of DCN, D13C14N, and D12C15N which where obtained from room temperature absorption measurements. Copyright 1999 Academic Press.

Intensities of Linear-Molecule Vibration-Rotation Transitions with |Deltak| = 2, with Applications to HCN, DCN, and HCCH

The Q branch of the 2nu22-0(0) band of the HCN molecule and an intensity asymmetry between the P and R branches have recently been observed by Maki, Quapp, Klee, Mellau, and Albert (J. Mol. Spectrosc. 185, 356-369 (1997)). Such intensity effects are associated with a contribution from the &Mtilde;22 terms of the effective dipole moment operator of Aliev and Watson. The quantum number dependence observed by Maki et al. (cited above) is in agreement with this contribution. A formula is derived for the coefficient of this term, and the value calculated from the anharmonic potential and dipole derivatives is in good agreement with the observed value for a particular choice of signs of the combination-band transition moments R12 and R32. For DCN the second derivatives of the dipole moment are not known, but a comparison with the HCN calculation suggests that the agreement is adequate in view of the relatively large uncertainty of the observed values. Similar intensity effects should occur in the (nu4 + nu5)2-0(0) band of HCCH, for which the coefficient is calculated to be similar in magnitude to that of the 2nu22-0(0) band of HCN. The intensity of the Sigma--Sigma+ component of (nu4 + nu5)-0 of HCCH, which should consist of only a Q branch, is also discussed; from present knowledge of the molecular parameters, it is expected to be much weaker than the Q branch of the Delta-Sigma+ component. Copyright 1998 Academic Press.