Scaling Factors for the Prediction of Vibrational Spectra. II. The Aniline Molecule and Several Derivatives M. ALCOLEA PALAFOX, 1 M. GILL, 1 N. J. NUNEZ, 1 V. K. RASTOGI, 2 LALIT MITTAL, 2,3 REKHA SHARMA 4 1 Departamento de Quimica-Fisica I, Facultad de Ciencias Quimicas, Universidad Complutense, Madrid 28040, Spain 2 Department of Physics, CCS University, Meerut 250 004, India 3 Department of Physics, Inderprastha Engineering College, Sahibabad, Ghaziabad, 201010, India 4 Department of Physics, Meerut College, Merrut, India Received 8 March 2004; accepted 28 September 2004 Published online 7 March 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.20416 ABSTRACT: The structure of aniline was studied by semiempirical, ab initio, and density functional methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational frequencies of aniline was determined. The normal modes were characterized by the magnitudes and direction of the displacement vectors. Three procedures were used to obtain the scaled frequencies, two of them new, using specific scale factors and scaling equations from the benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost-effective method and procedure of scaling was carried out. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem 103: 394 – 421, 2005 Key words: aniline; structural calculations; ab initio methods; density functional methods; vibrational wavenumbers; scaling procedures Introduction T he molecular prototype of an aromatic amine is the aniline. It has been the subject of several studies during the last two decades [1–5]. Its struc- ture has been reported theoretically using semi- empirical [6, 7], ab initio [1– 6, 8], and molecular mechanics [9] methods. It is also known experimen- tally in the gas phase from microwave spectroscopy [10, 11] and electron diffraction [12], and in solid state from X-ray crystallography [13]. However, in all these studies there is considerable estimated uncertainty in the nonplanarity of the amino group. The structure of this amino group is important for the chemical reactivity of aromatic amines, with great chemical and pharmacological applications. A Correspondence to: V. K. Rastogi; e-mail: v_krastogi@ rediffmail.com International Journal of Quantum Chemistry, Vol 103, 394 – 421 (2005) © 2005 Wiley Periodicals, Inc.