Microwave survey of the conformational landscape exhibited by the propeller molecule triethyl amine

Abstract

Conformational studies with quantum chemical methods yielded for the most stable conformer of triethyl amine a propeller-like structure belonging to the point group C(3), which corresponds to an oblate top. The microwave spectrum of this conformer with (14)N hyperfine splitting of all rotational transitions was assigned and molecular parameters were determined. The rotational constants were found to be A = B = 2.314873978(11) GHz, the (14)N quadrupole coupling constant χ(cc) = -5.2444(07) MHz. The observed spectrum could be reproduced within experimental accuracy. The standard deviation of a global fit with 48 rotational transitions is 1.5 kHz. The propeller-like structure seems to be energetically favorable and therefore also typical for related systems like triethyl phosphine, triisopropyl amine, tri-n-propyl amine, and tri-tert-butyl amine. Furthermore, the rotational transitions of two isotopologues, (13)C(2) and (13)C(5), could be measured in natural abundance and fitted with an excellent standard deviation. The C rotational constants could be determined to be 1.32681(96) GHz and 1.32989(18) GHz for the (13)C(2) and (13)C(5) isotopologues, respectively.

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:…”

“…[4] In these cases, the splittings arising from the methyl group could be observed, leading to a precise determination of the barrier heights. It is noteworthy that barriers to internal rotation of methyl groups at the end of an alkyl chain are often higher than 1000 cm À 1 , which usually inhibits the splittings to be resolved within the resolution of about 2 kHz of the spectrometer in use, as for example found in studies on triethyl amine [22] and diisopropyl ketone. [77] In contrast to methyl groups of longer alkyl chains, ethyl methyl groups show a more diverse range of barrier heights.…”

“…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).…”

Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry

“…Thisc ombination of method and basis set was chosen because it has produced reasonable agreement with experimental values in many previouss tudies on nitrogen-containing molecules. [8][9][10] In total, 36 different startingg eometries were created by varying two dihedral angles…”