Precise equilibrium structure determination of thiophene (c-C4H4S) by rotational spectroscopy—Structure of a five-membered heterocycle containing a third-row atom

Abstract

The rotational spectrum of thiophene (c-C4H4S) has been collected between 8 and 360 GHz. Samples of varying deuterium-enrichment were synthesized to yield all possible deuterium-substituted isotopologues of thiophene. A total of 26 isotopologues have been measured and least-squares fit using A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The resultant rotational constants (A0, B0, and C0) from each reduction were converted to determinable constants (A″, B″, and C″) to remove the impact of centrifugal distortion. The computed vibrational and electron mass corrections [CCSD(T)/cc-pCVTZ] were applied to the determinable constants to obtain semi-experimental equilibrium rotational constants (Ae, Be, and Ce) for 24 isotopologues. A precise semi-experimental equilibrium (reSE) structure has been achieved from a least-squares fit of the equilibrium moments of inertia. The combination of the expanded isotopologue rotational data with high-level computational work establishes a precise reSE structure for this sulfur-containing heterocycle. The CCSD(T)/cc-pCV5Z structure has been obtained and corrected for the extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born–Oppenheimer correction. The precise reSE structure is compared to the resulting “best theoretical estimate” structure. Several of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the reSE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.

Determination of the semiexperimental equilibrium structure of 2-acetylthiophene in the presence of methyl internal rotation and substituent effects compared to thiophene

“…Firstly, the structure reported by Orr et al 63 is in very good agreement with our r SE e structure, confirming the fact that it is essential to use accurate ground state rotational constants and that the structures of relatively rigid molecules are not sensitive to systematic errors of the rovibrational corrections. 64 Whereas the general agreement is rather pleasing, there are small differences of about 0.0008 Å between the most accurate structure BTE (best theoretical estimate) in Table 7 and the r SE e structure determined in Orr et al 63 for the C2–H2 and C2C3 bond lengths. These differences, although small, are larger than the standard deviations of the parameters.…”

“…After the completion of our work, a new semiexperimental structure of thiophene was published 63 which we also listed in Table 7. This new structure was obtained with the help of accurate ground state rotational constants of 24 isotopologues and a rovibrational correction calculated at the CCSD(T)/cc-pCVTZ level of theory.…”

Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

“…The planarity of the thiophene ring has been established in the literature. [9,54] Therefore, the conformational landscape of 2A3MT is only influenced by the orientation of the two substituents attached to the ring. As the rotation of the methyl group about the C3-C9 bond does not create new conformers, only the acetyl group has to be rotated for a conformational search.…”

“…Each vibrational level is three-fold degenerated, and therefore, no torsional splitting is observed in the spectrum. [4] The molecule can be considered as a rigid rotor, with thousands of examples previously studied using high resolution rotational spectroscopy, stretching several important subjects from fundamental interest, [5][6][7] structure determination, [8][9][10] astrophysics, [11][12][13] biomolecules, [14][15][16] chirality [17][18][19] to microsolvation [20][21][22] and complexes. [23,24] A barrier of zero, on the other hand, describes a free internal rotor where all energy levels except the first one are two-fold degenerated, as shown, e. g., for the methyl group in CH 3 À C�CÀ CF 3 .…”

Extending the Applicability of the Semi-experimental Approach by Means of “Template Molecule” and “Linear Regression” Models on Top of DFT Computations

“…In the framework of vibrational perturbation theory to the second order (VPT2), 46 these are expressed as where the α r i constants are the vibration–rotation interaction constants, i denotes the inertial axis, and the sum runs over all r vibrational modes. The evaluation of the α r i values requires anharmonic force field calculations, 14 , 23 , 27 , 28 , 32 which—in the present work—have been carried out using the global-hybrid B3LYP functional 47 , 48 in conjunction with the partially augmented double-ζ jun-cc-pVDZ basis set. Hereafter, this level of theory is shortly referred to as B3.…”