Isomer selective IR-UV depletion spectroscopy of 4-fluorotoluene-NH3: evidence for [small pi]-proton-acceptor and linear hydrogen-bonded complexes

The 4-fluorotoluene-ammonia van der Waals complex has been studied using IR-UV depletion and hole-burning spectroscopies with assignments supported by ab initio and DFT calculations of ground state binding energies and intramolecular vibrational frequencies. The experimental IR-UV depletion and hole-burning spectra presented here provide unequivocal empirical evidence that the 4FT-NH3 complex exists in two almost equally stable conformational forms. Both isomers contribute intensity to the experimental R2PI spectrum with one responsible for bands appearing to the red of the 4FT band origin position, and the other for those appearing immediately to the blue. On the basis of comparison of computed NH stretching frequencies with those obtained from the IR-UV spectra, the red-shifted bands are assigned to a [small pi]-proton-acceptor complex featuring an NH[small pi]-hydrogen bond, and the blue-shifted bands are assigned to an in-plane [sigma]-complex in which the ammonia binds in the plane of the ring forming a double-hydrogen-bonded six-membered ring with the fluorine atom. Ground state interaction energies computed at the M06-2X/cc-pVTZ level were found to compare favourably with those obtained at the CCSD(T) CBS level, although the former resulted in overbinding of the [small pi]-complex compared with the in-plane conformer, a characteristic shared with MP2 level calculations. The observation of a [small pi]-complex in addition to a [sigma]-complex is consistent with the conclusion that the electron-donating power of the methyl group is sufficiently large to counter-balance the electron-withdrawing power of the fluorine atom.