Anharmonic Vibrational Frequency Shifts upon Interaction of Phenol(+) with the Open Shell Ligand O 2 . The Performance of DFT Methods versus MP2 Vancho Kocevski and Ljupc ̌ o Pejov* Institute of Chemistry, Faculty of Science, “Sts. Cyril and Methodius University”, P.O. Box 162, 1001 Skopje, Republic of Macedonia ABSTRACT: Anharmonic vibrational frequency shifts of the phenol(+) O-H stretching mode upon complex formation with the open-shell ligand O 2 were computed at several DFT and MP2 levels of theory, with various basis sets, up to 6-311++G- (2df,2pd). It was found that all DFT levels of theory signi- ficantly outperform the MP2 method with this respect. The best agreement with the experimental frequency shift for the hydrogen-bonded minimum on the potential energy surfaces was obtained with the HCTH/407 functional (-93.7 cm -1 theoretical vs -86 cm -1 experimental), which is a significant improvement over other, more standard DFT functionals (such as, e.g., B3LYP, PBE1PBE), which predict too large downshifts (-139.9 and -147.7 cm -1 , respectively). Good agreement with the experiment was also obtained with the mPW1B95 functional proposed by Truhlar et al. (-109.2 cm -1 ). We have attributed this trend due to the corrected long-range behavior of the HCTH/407 and mPW1B95 functionals, despite the fact that they have been designed primarily for other purposes. MP2 method, even with the largest basis set used, manages to reproduce only less than 50% of the experimentally detected frequency downshift for the hydrogen-bonded dimer. This was attributed to the much more significant spin contamination of the reference HF wave function (compared to DFT Kohn-Sham wave functions), which was found to be strongly dependent on the O-H stretching vibrational coordinate. All DFT levels of theory outperform MP2 in the case of computed anharmonic OH stretching frequency shifts upon ionization of the neutral phenol molecule as well. Besides the hydrogen-bonded minimum, DFT levels of theory also predict existence of two other minima, corresponding to stacked arrangement of the phenol(+) and O 2 subunits. mPW1B95 and PBE1PBE functionals predict a very slight blue shift of the phenol(+) O-H stretching mode in the case of stacked dimer with the nearly perpendicular orientation of oxygen molecule with respect to the phenolic ring, which is entirely of electrostatic origin, in agreement with the experimental observations of an additional band in the IR photodissociation spectra of phenol(+)-O 2 dimer [Patzer, A.; Knorke, H.; Langer, J.; Dopfer, O. Chem. Phys. Lett. 2008, 457, 298]. The structural features of the minima on the studied PESs were discussed in details as well, on the basis of NBO and AIM analyses. 1. INTRODUCTION Rationalization and in-depth understanding of a number of phenomena in contemporary physical sciences has become almost impossible without a solid theoretical background in certain cases. One of the most exploited issues in physical chemistry nowadays is the problem of noncovalent bonding between molecular species. 1 The reason for this lies in the fact that clusters built up by noncovalently bonded molecular species are typical model systems that bridge the gap between isolated molecules and the corresponding condensed phases formed by them. 1 Studies of molecular clusters offer the exceptional possi- bility to study the evolution of various properties going from isolated molecules, through their increasingly larger associates, and finally - the condensed phases. Among the vast variety of noncovalent intermolecular interaction types, those involving aromatic molecules are of exceptional importance, especially in relation to chemical and biological recognition phenomena. 2 For example, clusters of phenol with various molecules (ligands) are typical noncovalently bonded systems which offer an excep- tional possibility to study two competing types of noncovalent interactions - hydrogen bonding and stacking (π-bonding). 3-19 This possibility is due to the fact that the ligand molecules can interact either with the aromatic ring or with the acidic hydrogen atom from the OH group. In the former case, the non- covalent interaction is of π-bonding type (stacking), whereas in the latter it could be either a hydrogen bond (conventional or unconventional, depending on the ligand type and the X-H oscillator) or even electrostatic interaction (dipole-quadrupole, quadrupole-quadrupole, etc.). Whereas the π-bonding inter- action is favored in the case of nonpolar ligands (such as, e.g., rare gas atoms, methane, etc.), due to the predominance of dispersion forces between the ligand and the highly polarizable Received: October 12, 2011 Revised: January 25, 2012 Published: January 25, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 1939 dx.doi.org/10.1021/jp209801s | J. Phys. Chem. A 2012, 116, 1939-1949