Ultrasonic Relaxation Measurements in Aqueous Solution and Molecular Orbital Calculation on Imipramine

Abstract

Ultrasonic absorption coefficients have been measured in aqueous solution of imipramine {3-(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethylpropan-1-amine} in the frequency range of 0.8-220 MHz at 25 °C. The frequency dependences of the observed absorption was characterized by a Debye-type relaxational equation with two relaxation frequencies, although only one relaxation had been observed in aqueous solutions of the related molecule amitriptyline. Both of the relaxation frequencies in imipramine solutions were found to be independent of the solute concentration and the amplitudes of the relaxational absorptions increase linearly with increasing solute concentration. It was therefore concluded that these two relaxations are associated with unimolecular reactions, such as a structural change due to rotational motions of the bond in the specified group in the imipramine molecule. To analyze quantitatively the source of the relaxations, semiempirical molecular orbital methods have been applied to determine the standard enthalpy of formation of the imipramine molecule at various dihedral angles around the bonds in the alkylamine side chain. According to the results, only one rotational motion of carbon-carbon bond in the side chain was found to be appropriate and the three minima of the standard enthalpy of formation was obtained as a function of the rotational angle. At the three minimum positions, the values of the standard enthalpy of formation are almost the same. With the assumptions (a) that rotational motion is not accompanied by a volume change of the reaction and (b) that the standard free energy change is close to the difference in the values between the standard enthalpies of formation, the equilibrium constants for the rotational isomerization have been calculated to be near unity. Hence, the forward and backward rate constants of the isomerization reactions are nearly the same. If one assumes that there are two kinds of rotational motions in one bond of the molecule, one proceeds with a rate constant on the order of 10(8) s(-1), whereas the other with a rate constant on the order of 10(6) s(-1). The faster and slower processes are also distinguished by the height of the standard enthalpy of formation.

Thermo-Acoustical Study of Biologically Active 1,1’-Bis(3-Methyl-4-Carboxyethylphenoxy) Cyclohexane at Four Different Temperatures

“…Published by Academic Open Access LTD, 2015 Recently ultrasonic sound radiations are used in synthetic organic chemistry, which cause a decrease of reaction time, increase of yield, lower reaction temperature, and avoidance of phase transfer catalysis [5][6][7]. Further ultrasonic sound velocity measurements have been used to study the nature of molecular interactions in various liquid mixtures and solution of organic and inorganic compounds [8][9][10], solutions of polymers [11,12], pharma materials [13,14] and ionic interactions in electrolytes solutions [15,16].…”

Thermo-Acoustical Study of Biologically Active 1,1’-Bis(3-Methyl-4-Carboxyethylphenoxy) Cyclohexane at Four Different Temperatures

“…Further ultrasonic sound velocity measurements have been used to study the nature of molecular interactions in various liquid mixtures and solution of organic and inorganic compounds [8][9][10], solutions of polymers [11,12], pharma materials [13,14] and ionic interactions in electrolytes solutions [15,16].…”