Indian Journal of Pure & Applied Physics Vol. 43, December 2005, pp. 911-917 Density functional theory and FTIR spectroscopic study of carboxyl group Medhat Ibrahim 1 , Abdallah Nada 2 , & Diaa Eldin Kamal 1 1 Spectroscopy Department, National Research Center, Dokki, Cairo, Egypt 2 Cellulose and Paper Department, National Research Center, Dokki, Cairo, Egypt Received 29 March 2005;revised 7 September 2005;accepted 30 September 2005 Both molecular modelling and FTIR have been used to study carboxyl group among acetic acid, potassium and sodium acetate, glycine, sodium salicylate, salicylic acid and cellulose acetate. Molecular modelling was used to study formic acid, carboxylic acids R-COOH, monovalent alkali carboxylate CH 3 COO-M, alanine, benzoic acid as well as naphthalene. Each structure was optimized using VWN/DZVP then vibrational spectra were further calculated at the same level of theory. The comparison between both calculated and experimental spectra showed good agreement with each other. Except for formic acid and free carboxyl, the characteristic band is shifted towards lower frequency. Keywords: FTIR spectroscopy, Density functional theory, Carboxyl group, Cellulose acetate IPC Code: G01J3/28 1 Introduction Studying carboxyl group (COOH) and its interactions is very important in many areas of science: such as surface science 1-2 , electrochemistry 3-4 , and biology 5,6 . In environment, COOH of humic acid plays a crucial role in speciation, transport and deposition of metal ions 7-10 . It is one of the important groups leading to the reactivity of humic substances 11-13 . Furthermore, trace metals could interact with humic substances as a result of electrostatic attraction and/or formation of a chelate structure to a charged COO group 12 . COOH of both formic and carboxylic acids possess potentially two proton binding sites namely OH and C=O groups. Proton bound clusters are known to form hydrogen bounded networks 14 . Many proton bounded clusters have been investigated experimentally 15,16 as well as througth molecular modelling 17,18 . The existence of COO ν as around 1535 cm -1 and ν s around 1424 cm -1 is a characteristic feature of Ca ++ bounded Calmodulin and Parvalbumin 19-21 . The spectra of K, Na, Mg and Ca caroxylate were studied earlier, experimentally as well as via molecular modelling 22 . Molecular modelling was used to study COOH among different structures. The model B3LYP/6-31G(d,P) was used to obtain the vibrational spectra of carboxyl in case of acetic acid, polyacrylic acid as well as benzoic acid 23 . Ab initio calculations were used to study the interface between titanium oxide and amino acid in solution 24 . Both ab initio and Density Functional Theory (DFT) were used to predict the 1:1 complexes of formic acid with pyrrole or imazole 25 . Both FTIR and DFT were used to study the spectra of monomeric glycolic acid 26 . Twelve triple complexes of nine adenine tautomers with carboxylate ion of acetic acid and sodium ion were studied by DFT method 27 . Both infrared and ab initio of the hydrogen bonding between formic acid and water were studied. The complex formation resulted in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid 28 . Vibrational mode analysis on mechanism of the pyrolysis of formic acid in the gas-phase catalyzed by water dimer or formic acid itself was proposed 29 . Infrared spectra of bicyclic and tricyclic amidine derivatives of alanine were computed. The correlation between the calculated and experimental vibrational frequencies was characterized by the coefficients of 0.9997 for DFT methods; and 0.9992 for HF (Ref.30). The geometric and electronic structure of 1,8-bis(dimethylamino)-4-cyano-naphthalene and its mono-protonated cation have been investigated by DFT and time dependent density functional theory methods 31 (TD-DFT). Both IR and Raman spectroscopic studies of salicylic and salicylate derivatives in aqueous solution were conducted 32 , and results were compared with DFT. The intramolecular hydrogen bonding of salicylic acid was further studied 33 . Both kinetic and chemical modification analysis suggested that carboxylate and carboxyl groups are involved directly in the enzymatic reaction of -glucosidases 34-35 . Enzyme avoids