A rotational investigation of the three isomeric forms of cyanoethynylbenzene (HCC-CH-CN): benchmarking experiments and calculations using the "Lego brick" approach

We report the study of three structural isomers of phenylpropiolonitrile (3-phenyl-2-propynenitrile, C 6 H 5 -C 3 N) containing an alkyne function and a cyano group, namely ortho -, meta -, and para -cyanoethynylbenzene (HCC-C 6 H 4 -CN). The pure rotational spectra of these species have been recorded at room temperature in the millimeter-wave domain using a chirped-pulse spectrometer (75-110 GHz) and a source-frequency modulation spectrometer (140-220 GHz). Assignments of transitions in the vibrational ground state and several vibrationally excited states were supported by quantum chemical calculations using the so-called "Lego brick" approach [A. Melli, F. Tonolo, V. Barone and C. Puzzarini, J. Phys. Chem. A, 2021, 125 , 9904-9916]. From these assignments, accurate spectroscopic (rotational and centrifugal distortion) constants have been derived: for all species and all observed vibrational states, predicted rotational constants show relative accuracy better than 0.1%, and often of the order of 0.01%, compared to the experimental values. The present work hence further validates the use of the "Lego brick" approach for predicting spectroscopic constants with high precision. Benchmarking experiments and calculations using the "Lego brick" approach on cyanoethynylbenzene isomers.