The heavy atom structures and ³³S quadrupole coupling constants of 2-thiophenecarboxaldehyde: insights from microwave spectroscopy

Abstract

We report on the structures of two conformers of 2-thiophenecarboxaldehyde as obtained using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemical calculations. The microwave spectrum was recorded using two spectrometers operating in the frequency ranges of 2.0 GHz to 26.5 GHz and 26.5 GHz to 40.0 GHz. The spectra of all singlysubstituted heavy atom isotopologues 13 C, 18 O, and 34 S in their natural abundances could be measured and assigned to determine the experimental gas-phase substitution rs and semi-empirical re SE structures of the most abundant conformer. The spectrum of the 33 S isotopologue with its nuclear quadrupole coupling hyperfine structure was analyzed, yielding the complete quadrupole tensor with aa = 22.63799(76),  = bb  cc = 18.4892( 14), and ab = 12.002(33) MHz.Quantum chemical calculations for the electric field gradient tensor including corrections with a calibration factor determined from 27 previous studies on the 33 S species predicted the 33 S nuclear quadrupole coupling constants of 2-thiophencarboxaldehyde with high quality. The experimental results are used to map the observed rotational constants to the corresponding molecular structure obtained from quantum chemical calculations, which predicted two conformers with an energy difference of about 6 kJmol 1 at the MP2/6-311++G(d,p) level of theory. Insight into the conformational stability of aromatic heterocyclic carboxaldehydes and bond situations of the sulfur atom extracted from the hyperfine structure of the 33 S nucleus are discussed within the frame of the current literature. This work provides an important contribution to the study and characterization of sulfur-containing volatile organic compounds.

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan‐2‐one

“…0.00 [c] 1.21 [e] 1.76 [c] 0.00 [e] [a] Rotational constants obtained from the XIAM(1Top) fit, see ing rotational constants for assignment purposes in a variety of molecules, as shown in many of our own investigations [20][21][22] and studies in the literature. [23][24][25] Nevertheless, for methyl nalkyl ketones, [2][3][4] especially their C 1 conformers, the accuracy of the rotational constants predicted at this level does not satisfy the experimental requirements which are crucial for a successful assignment of the microwave spectrum (see Section 3).…”

“…Nevertheless, a good agreement between X e and X 0 is often accidentally achieved at the MP2/6-311 + + G(d,p) level due to error compensation. [20][21][22][23][24][25] This does not apply for the C 1 conformers of methyl alkyl ketones. Good starting values for the rotational constants of octan-2-one can instead be determined with a semi-empirical correction using experimental data of other methyl alkyl ketones, like for example heptan-2-one, [4] and the following equation:…”

“…The MP2/6‐311++G(d,p) level of theory has been chosen for the geometry optimizations, since it has provided a good balance between speed of calculation and reliability in predicting rotational constants for assignment purposes in a variety of molecules, as shown in many of our own investigations and studies in the literature . Nevertheless, for methyl n ‐alkyl ketones, especially their C 1 conformers, the accuracy of the rotational constants predicted at this level does not satisfy the experimental requirements which are crucial for a successful assignment of the microwave spectrum (see Section 3).…”

“…In general, quantum chemical calculations yield equilibrium rotational constants X e (with X = A , B , C ), while with microwave spectroscopy rotational constants of the vibrational ground state X 0 are obtained. Nevertheless, a good agreement between X e and X 0 is often accidentally achieved at the MP2/6‐311++G(d,p) level due to error compensation . This does not apply for the C 1 conformers of methyl alkyl ketones.…”

2-Propionylthiophene: planar, or not planar, that is the question

“…[12][13][14][15][16] For the carbonyl group, there are two possible are stabilized by π-conjugations. 17 Therefore, it was almost obvious to assume that four stable conformers of 2PT would exist with two possible orientations of the ethyl group (Cs and C1) and two of the carbonyl group (syn and anti). We were interested in testing and validating this assumption, as well as in studying which conformers are present in the experimental spectrum.…”