Internal rotation and equilibrium structure of 2-nitropropane from gas electron diffraction and quantum chemistry

Abstract

In this paper, the equilibrium structural parameters of the 2-nitropropane molecule and the barrier of internal rotation of the nitrogroup are determined from the gas electron diffraction data, with the use of quantum chemistry calculations and experimental vibrational frequencies, in the framework of the large-amplitude motion model for internal rotation. Quantum chemistry calculations at the MP2 and B3LYP levels of theory with various Pople and Dunning basis sets unambiguously predict the same minimum energy molecular conformation, with relatively close values of internal rotation barrier (375–525 cm −1). The results of present analysis show that the minimum of the potential function of the nitrogroup internal rotation is located in syn-H position when one of the oxygen atoms eclipses hydrogen atom that does not belong to any of CH 3 groups (dihedral angle H–C–N–O is zero). It has also been found that internal rotation is hindered, with the barrier height in the range of 220–560 cm −1 (0.6–1.6 kcal/mol) with the most probable value near 380 cm −1 (1.1 kcal/mol). The main equilibrium structure parameters in syn-H configuration are as follows (values in parentheses correspond to 3 times standard deviations): r e(C–C) = 1.516(5) Å, r e(C–N) = 1.501(5) Å, r e (N O) = 1.225(4) Å, ∠ C–C–N = 108.7 ( 1.0 ) ° , ∠ O N O = 124.8 ( 0.4 ) ° . We also provide thermally averaged parameters for comparison with the results of previous studies.