Journal of Molecular Spectroscopy最新文献

It is sometimes difficult to determine the structure of some molecules because the optimization using standard ab initio methods (coupled-cluster with single, double, and perturbative triples [CCSD(T)] level) does not give the correct result and the experimental and semiexperimental methods are not accurate because the system of normal equations of the least-squares method is ill-conditioned. In such a case, it may be still possible to derive an accurate equilibrium structure in the following way: the experimental rotational constants are compared to those obtained at the CCSD(T) level, the latter being corrected to take into account the rovibrational correction (and, if necessary, the electronic correction). Extrapolating (or interpolating) the rotational constants calculated with different basis sets (e.g. cc-pwCVTZ and cc-pwCVQZ) towards the experimental values as a function of the bond lengths and angles permits to obtain an accurate equilibrium structure. This method is first tested on two molecules for which the multireference effects are important: O₃ and HOON. It is then, applied to molecules with a weak N–X bond (HONO, FNO, ClNO, FNO₂, and N₂O) for which the single reference CCSD(T) method gives bonds that are too short. The results are compared to the experimental and semiexperimental equilibrium structures. As a further check, the structure of ClNO is calculated at the CCSDTQ level and the structures of FNO and ClNO are calculated at the MRCI-F12 level. From a comparison of the different results, it appears that the accuracy of the proposed method is better than 0.002 Å for the bond lengths and 0.3° for the angles.

We prepared a sample of oxirane doubly deuterated at one C atom and studied its rotational spectrum in the laboratory for the first time between 120 GHz and 1094 GHz. Accurate spectroscopic parameters up to eighth order were determined, and the calculated rest frequencies were used to identify $c$-CD${}_2$CH${}_2$O tentatively in the interstellar medium in the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293-2422. The $c$-CD${}_2$CH${}_2$O to $c$-C${}_2$H${}_4$O ratio was estimated to be $\sim$0.054 with $T_{\mathrm{rot}}=125$ K. This value translates to a D-to-H ratio of $\sim$0.16 per H atom which is higher by a factor of 4.5 than the $\sim$0.036 per H atom obtained for $c$-C${}_2$H${}_3$DO. Such increase in the degree of deuteration referenced to one H atom in multiply deuterated isotopologs compared to their singly deuterated variants have been observed commonly in recent years.

Rotational transitions of propynal (HCCCHO) have been measured in the 150–900 GHz region by millimeter wave spectroscopy and in the far infrared region by high resolution FTIR spectroscopy using a synchrotron source. For the parent isotopologue, assignment of MMW transitions up to very high quantum numbers (J = 100, K_a = 25) reveals evidence of extensive perturbations in the ground vibrational state due to Fermi-asymmetry resonance with an excited vibrational state. A fit to nearly 3000 ground state transitions yields effective constants that are suitable for describing relatively unperturbed rotational levels up to K_a = 13. Over 1000 transitions were assigned and fitted for each singly substituted ¹³C species and nearly 400 transitions for the ¹⁸O variant. Re-analysis of literature data on deuterated species, aided by centrifugal distortion constants from hybrid density functional theory calculations at the B2PLYP/aug-cc-pVTZ level, provides a further set of rotational constants. This allows determination of a R_m⁽²⁾ structure for propynal with the following geometry: r(C≡C) = 1.2066(15), r(CC) = 1.4486(14), r(CO) = 1.2087(10), r(C­H ald) = 1.1069(8), r(C­H acet) = 1.0578(13), θ(CCC) = 176.71(22), θ(OCC) = 123.23(7), θ(HCC ald) = 114.43(31), θ(HCC acet) = 178.45(16). A new R_s structure was also derived.

The rotational spectrum of the syn conformer of thiobenzoic acid (C₆H₅COSH) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectra of all singly-substituted isotopologues involving ¹³C, as well as the ¹⁸O, ³³S, ³⁴S, and −SD derivatives have also been recorded. The isotopic data have been used to derive structural parameters of the molecular frame. The results are compared with density functional theory calculations at theM06-2X/6-311++G(d,p) level and show excellent agreement. The molecule is planar at the equilibrium geometry, and calculations of the energy profile for out-of-plane torsion of the SH group are presented. The anti conformer was not observed, consistent with calculations which indicate that it lies 2.6 kcal/mol higher in energy than the syn form.

The rotational spectra of four substituted benzoic acids, i.e., para-aminobenzoic acid, para-nitrobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid were recorded with a chirp-pulse Fourier transform microwave spectrometer and analysed in terms of rotational constants and, where applicable, nuclear quadrupole coupling constants and tunnelling motions. In all instances, the experimentally identified conformer contains a carboxylic acid group in the cis-arrangement. In the case of para-hydroxybenzoic acid, spectra of two conformers were observed which differ in the orientation of the para-OH group. Quantum chemical calculations at the B3LYP-D3BJ/def2-TZVP and B2PLYP-D3BJ/cc-pCVTZ levels show that the trans-conformers are about 23 kJ mol⁻¹ higher in energy than the global minimum cis-conformers. In general, the calculated rotational constants are in good agreement with the experimental derived ones. Agreement between calculated and experimental ¹⁴N and ^35/37Cl nuclear quadrupole coupling constants is somewhat worse and necessitated the use of the B2PLYP functional in the case of para-nitrobenzoic acid to reduce the discrepancy. Conversion barriers between possible conformers were calculated to explain the absence of tunnelling splittings and/or higher energy conformers in the experimental spectra. An analysis of the electron density distribution was used to rationalize the calculated out-of-plane excursions of the carboxylic acid group in the trans-conformers.

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the X${}^1{\Sigma }_g^{+}$ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H₂S, HDS and D₂S are used to produce vibrationally hot H₂, HD and D₂. The wave function density at large internuclear separation is excited via two-photon transitions in the F${}^1{\Sigma }_g^{+}$ - X${}^1{\Sigma }_g^{+}$ system to probe ro-vibrational levels in the first F${}^1{\Sigma }_g^{+}$ outer well state of gerade symmetry. Combining with accurate knowledge of the X${}^1{\Sigma }_g^{+}$ ($v,J$) levels from advanced ab initio calculations, energies of rovibrational levels in the F${}^1{\Sigma }_g^{+}$ state are determined. For the H₂ isotopologue a three-laser scheme is employed yielding level energies at accuracies of $4\times 10^{-3}$ cm⁻¹ for F($v=0,J$) up to $J=21$ and for some low $J$ values of F($v=1$). A two-laser scheme was applied to determine level energies in H₂ for F($v=0-4$) levels as well as for various F levels in HD and D₂, also up to large rotational quantum numbers. The latter measurements in the two-laser scheme are performed at lower resolution and the accuracy is strongly limited to 0.5 cm⁻¹ by ac-Stark effects. For H₂ a new quasibound resonance X