Brenda P. Winnewisser

Interactive Loomis-Wood assignment programs

Abstract The traditional Loomis-Wood diagram forms the basis of interactive assignment programs implemented on a PC. These programs allow fast and convenient identification of bands or subbands in line position lists of rotationally resolved band spectra.

HCNO as a semirigid bender

Abstract The semirigid bender Hamiltonian [Bunker and Landsberg, J. Mol. Spectrosc. , 67 , 374–385 (1977)] is used to fit the rotation-vibration energy level separations in the fulminic acid (HCNO) molecule. The allowance made in the model for the variation of the CH and CN bond lengths with the HCN bending angle proves to be very important, and as well as achieving a good fit we are able to make a detailed investigation of the shape of the HCN bending potential function. From the results we conclude that the equilibrium structure of HCNO is linear but that excitation of the ν 1 or ν 2 stretching vibrations gives rise to an effective HCN bending potential function having its minimum at a nonlinear configuration. Even in the ground state the zeropoint vibrational contributions from ν 1 and ν 2 to the effective HCN bending potential give a small barrier (11.5 cm −1 ) to linearity, and we determine that the zero-point HCN bending vibrational amplitude is ±34°.

High resolution wavenumber standards for the infrared (Technical Report)

The calibration of high resolution infrared spectra is generally more precise than accurate. This is the case even when they are recorded with Fourier transform interferometers. The present document aims at improving the accuracy of wavenumber measurements in the infrared by recommending a selection of spectral lines as wavenumber standards for absolute calibration in the range from about 4 to about 7000 cm(-1). The uncertainties of these wavenumber standards range from 4+/-1x10(-3) to +/-1x10(-6) cm(-1). Sources of frequency standards, on which the wavenumber determinations are based, are also given.

The bending-rotation spectrum of fulminic acid and deuterofulminic acid☆☆☆

Abstract A quasilinear molecular model is needed to account for the infrared absorption spectrum of HCNO and DCNO in the spectral region from 100 cm −1 to 1000 cm −1 . The observed systems of infrared bands arising from the ν 5 vibrational manifold have all been assigned. The rotational structure of the absorption bands at 225 cm −1 , 275 cm −1 , 315 cm −1 , and 317 cm −1 for HCNO has been resolved using a Fourier spectrometer. The rotational constants and the band centers have been determined for the above bands, which represent the transitions (0000 0 1 1 ) c ←0000 0 0 0 (0000 0 2 2 ) c,d ←(0000 0 1 1 ) c,d both components (0000 0 3 3 ) c,d ←(0000 0 2 2 ) c,d 0000 0 2 0 ←(0000 0 1 1 ) c . By means of the Ritz combination principle the infrared transitions could be used to build up the vibrational energy level scheme of the ν 5 vibrational mode for HCNO and DCNO. The data are only reconcilable with a potential function for ν 5 which exhibits a low barrier opposing linearity. Preliminary values of the potential parameters were obtained using different approximate theoretical approaches. A reinterpretation of the r 8 structure parameters of fulminic acid in the light of the quasilinear model leads to an explanation of the extraordinarily short CH internuclear distance of 1.027 A as the projection of a CH bond length of 1.060(5) A upon the heavy-atom axis. The isotopic shift upon deuteration observed in the infrared data indicate that the ν 5 fundamental vibration is primarily an HCN bending motion. The ν 4 fundamental vibration (skeletal bending motion) of HCNO is located at 537 cm −1 and does not exhibit any hot band structure which would be indicative of a perturbed potential function.

Matrix-isolation infrared investigation of the flash vacuum thermolysis of norbornadienone azine

Abstract The flash vacuum thermolysis of norbornadienone azine over the temperature range 300 to 800°C was investigated by matrixisolation IR methods. The following thermolysis products could be identified: isocyanogen, cyanogen, benzene, benzonitrile and hydrogen cyanide. A tentative assignment of the infrared-active ν 2 stretching vibration of diisocyanogen, CNNC, in an argon matrix is made. The peak positions of four fundamentals (ν 1 to ν 4 ) of isocyanogen, CNCN, and of two stretching fundamentals (ν 1 and ν 2 ) of 13 CNCN, CN 13 CN, C 15 NCN and CNC 15 N in an argon matrix were measured. Combination bands yielded a value for ν 5 of CNCN. A molecular force field for CNCN was determined.

Vibration-rotation interaction in HCNO caused by accidental resonances and enhanced by the quasilinearity of the molecule☆☆☆

Abstract The recent assignment of the vibrational spectrum of the quasilinear molecule HCNO revealed several near coincidences between vibrational energy levels involving the two bending modes, ν4 (skeletal bending mode) and ν5 (HCN-bending mode), and the lowest-lying stretching mode, ν3 (NO stretching mode). By considering the correlation between the energy levels of a linear and a bent molecular model of HCNO, it is seen that resonance interactions which would be of third or higher order in a linear molecule Hamiltonian would be of first or second order in the Hamiltonian of a bent molecule, and thus might be significant in the quasilinear molecule HCNO. In this way we were able to identify the type of observed interaction occurring between three pairs of nearly coincident levels, (00010, 00002), (00020, 00012), and (00100, 00004). Anomalous centrifugal distortion effects had been observed and reported earlier for the pure rotational transitions arising from molecules in the 00010, 00020, and 00002 levels. Rotational transitions arising from molecules in the 00004 and 00100 vibrational states of HCNO and the 00100 state of DCNO are reported here for the first time. For two pairs of levels, (00010, 00002) and (00100, 00004), we could determine the magnitude of the coefficients of the interaction matrix elements from an analysis of the centrifugal distortion effects.

Molecular structure and spectroscopic properties of carbodiimide (HNCNH)

Abstract Ab initio molecular orbital calculations, including geometry optimizations up to the level of MP3/6-31G**, have been carried out for carbodiimide (HNCNH). The best predicted structure for carbodiimide, which includes empirical adjustments to some geometrical parameters, has r (NH) = 1.010 A, r (CN) = 1.224 A, ∠ HNC = 119.7°, ∠ NCN= 170.8° and ∠HN…#NH=89.6°. The calculated HNC angle shows a large variation between different levels of theory. Similar behaviour is found to occur for the related molecules, isocyanic acid (HNCO) and ketenimine (HNCCH 2 ), but not for methanimine (HNCH 2 ). The experimental observation that carbodiimide is one of the most nearly accidentally symmetric top molecules known has been confirmed. Stereomutation in carbodiimide is predicted to occur via internal rotation rather than pure inversion, but substantial (≈ 13°) opening of the HNC bond angle accompanies the torsional motion. The vibrational frequencies for carbodiimide have been calculated at the HF/6–31 G* level and empirically adjusted to yield predicted experimental values.

The spectrum of CH3OH between 200 and 350 cm−1: Torsional transitions and evidence for state mixings

Abstract In the framework of our systematic investigation of the infrared and far infrared spectrum of CH3OH by high-resolution Fourier transform spectroscopy, we present a catalog of 8800 absorption lines between 200 and 352 cm−1. The Taylor expansion coefficients for evaluating the energies of the levels involved in the transitions are also given. All of the lines occurring in the 200–352 cm−1 region correspond to transitions between torsionally excited states. Some forbidden lines (Δn ≠ 0; ΔK = 0), indicating state mixings, have been found.

Millimeter wave spectrum of acetonitrile oxide, CH3CNO, in the ν10 vibrational manifold: The ground state and the state v10 = 1

Abstract The pure rotational spectrum of CH 3 CNO was measured in the frequency range 75 to 230 GHz. For the ground state, transitions were measured for J between 9 and 28 and for K from 0 to 12. In the v 10 = 1 state the measurements range from J = 0 to 19 and from K = 0 to 11. Numerous perturbations are observed, apparently due to accidental resonances with levels in other vibrational states. The contributions due to Δ K = 2, Δ l = 2 matrix elements ( l -type resonance and l -type doubling) are accounted for by matrix diagonalization, and the effects due to accidental resonances are presented graphically.

Millimeter wave rotational spectrum of deuterofulminic acid, DCNO, in excited vibrational states☆☆☆

Abstract Rotational transitions of DCNO in the vibrational states 00002, 00003, 00020, and 00011 have been measured and analyzed. These results complete the presentation of assigned millimeter wave transitions of DCNO. The analysis of rotational l-type resonance in the 00002 and 00020 states is more satisfactory in the case of DCNO than in that of HCNO due to the absence of accidental resonances. The values obtained for the vibrational anharmonicity constant g55 for 00002 and 00003 agree within experimental error with those found from the vibrational spectrum. An ambiguity in the assignment of the symmetry of the transitions in the four components of the 00011 vibrational state is discussed.