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On the NH stretch ν1 band of methyleneimine CH2NH
1Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.
Abstract:
Over four decades, the 3262.622(2) cm-1 band [Halonen and Duxbury, Chem. Phys. Lett. 118, 246-251 (1985)] of H2C=NH (methyleneimine or methanimine) has been cited as the ν1 NH stretch fundamental. In this article, the assignment is comprehensively reviewed for the first time, concluding that the band should instead be assigned to the CN stretching overtone 2ν4. The analysis of vibrational wavenumbers and transition dipole moments is guided by large basis set coupled-cluster anharmonic frequency calculations. A comparison with the vibrations of its perfluorinated analog (difluoromethanimine, F2C=NH) exemplifies that fc-CCSD(T)/ANO2 VPT2, as expected, is able to provide very accurate vibrational band centers for these semi-rigid molecules. As a by-product of the analysis, a significant reorientation of the ν1 transition dipole moment of methyleneimine by more than 100° away from the NH bond toward the nitrogen lone pair is found. MP2 fails to capture this effect, possibly due to a mild multireference character as suggested by a comparison with CCD, CCSD, and CCSD(T).
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