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On the Origin of Red and Blue Shifts of XH and CH Stretching Vibrations in Formic Acid (Formate Ion) and Proton Donor Complexes
Author(s) -
Tâme Parreira Renato Luis,
Galembeck Sérgio Emanuel,
Hobza Pavel
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200600435
Subject(s) - chemistry , antibonding molecular orbital , lone pair , formic acid , formate , electron transfer , electron donor , crystallography , electron density , ion , proton , hydrogen bond , density functional theory , proton coupled electron transfer , photochemistry , electron , atomic orbital , computational chemistry , molecule , organic chemistry , catalysis , physics , quantum mechanics
Complexes between formic acid or formate anion and various proton donors (HF, H 2 O, NH 3 , and CH 4 ) are studied by the MP2 and B3LYP methods with the 6‐311++G(3df,3pd) basis set. Formation of a complex is characterized by electron‐density transfer from electron donor to ligands. This transfer is much larger with the formate anion, for which it exceeds 0.1 e. Electron‐density transfer from electron lone pairs of the electron donor is directed into σ* antibonding orbitals of XH bonds of the electron acceptor and leads to elongation of the bond and a red shift of the XH stretching frequency (standard H‐bonding). However, pronounced electron‐density transfer from electron lone pairs of the electron donor also leads to reorganization of the electron density in the electron donor, which results in changes in geometry and vibrational frequency. These changes are largest for the CH bonds of formic acid and formate anion, which do not participate in H‐bonding. The resulting blue shift of this stretching frequency is substantial and amounts to almost 35 and 170 cm −1 , respectively.