Vibrational spectra of silatranes and germatranes XM(OCH2CH2)3N (X = F,Cl,H; M = Si,Ge). The problem of the theoretical prediction of condensed phase spectra

The structures of silatranes and germatranes XM(OCH2CH2)3N (X = F,Cl,H; M = Si,Ge) were optimized and their vibrational spectra were calculated at the B3LYP/aug-cc-pVDZ level of theory. Theoretical frequencies of vibrations perpendicular to the C3 axis (E type) are in good agreement with experimental values, while the axial vibrations (MX and M…N stretchings) demonstrate a significant discrepancy with experimental spectra recorded for the crystalline state. This discrepancy stems from the well-known difference in the MX and M…N bond lengths in gas and solid state. The force constant scaling procedure was used to compensate for this difference. As a result a set of scaling factors was refined for 1-Cl-germatrane (the unique atrane for which the distinction between A and E modes was experimentally established). This set was transferred to the theoretical force fields of other atranes, which provided a fair reproduction of their experimental frequencies. The analysis of the normal modes allowed us to assign the ν M…N mode to bands in the 180–270 cm−1 frequency range, although large contributions of these coordinates are in two other modes in the 450–500 cm−1 and 600–800 cm−1 frequency ranges.The frequencies of degenerate vibrations (with vectors perpendicular to the C3 axis) do not depend substantially on the axial atom (X and M) substitution, while those of A-type in the 200–700 cm−1 frequency range vary significantly.

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► The vibrational spectra are assigned using scaled force fields.
► Emphasis is placed on the attribution of modes.
► This involves stretching of the transannular X…N bond.