Elsevier

Vibrational Spectroscopy

Volume 50, Issue 2, 20 July 2009, Pages 307-311
Vibrational Spectroscopy

IR spectroscopic studies on the transmission of substituent effects to carbonyl and thiocarbonyl stretching frequencies in 4-substituted phenyl-4,5-dihydrobenzo [f] [1,4] oxazepin-3 (2H)-ones (thiones)

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Abstract

Single and dual substituent correlation analysis were applied to study transmission of substituent effects on IR carbonyl and thiocarbonyl stretching frequencies of 4-substituted phenyl-4,5-dihydrobenzo [f] [1,4] oxazepin-3 (2H)-ones (5an) and -thiones (6an). The substituent effects were estimated on the basis of results of the statistical analysis. The differences among the regression coefficients were discussed in terms of the relative importance of the substituent field and resonance effects. For a better understanding of the results, density functional theory (DFT) calculations were performed to determine the preferred geometry and to calculate the theoretical carbonyl and thiocarbonyl stretching frequencies.

Introduction

Infrared (IR) spectroscopy is a powerful tool for the quantitative study of natural and synthetic molecules [1]. IR spectroscopy can provide information about the nature, concentration and structure of samples at the molecular levels.

It is well demonstrated in the literature [2], [3], [4], [5], [6], [7], [8], [9] that infrared (IR) stretching frequencies are sensitive probes of electron-density distributions and are useful for studies on the transmission of electronic effects in organic molecules.

Compounds containing the carbonyl group present a strong absorption band in the infrared region, placed within 1928 and 1515 cm−1, referring to the stretching vibration of the Cdouble bondO bond, ν(Cdouble bondO). The terminal value of the force constants of this bond, minimize couplings with adjacent groups, while its polarizability turns it susceptible to the effects coming from its surroundings [10], [11]. Therefore, the accurate position of the Cdouble bondO group absorption band in the IR is result of the action of intermolecular and intramolecular factors, mainly of the electronic influence of substituent groups. Hence, variations in the absorption ν(Cdouble bondO) band position in the IR region can be considered as being the result of the polar effects of the substituent groups.

The benzoxazepines with carbonyl groups have shown anxiolytic [12], central depressant [13], analgesic [14], local anesthetic [15], anti-inflammatory [16] and antihistaminic [17] effects. So, it is expected that also the title compounds will exhibit various physiological effects. The aim of this study is the IR investigation on transmission of electronic effects by substituents in a series of 4-substituted phenyl-4,5-dihydrobenzo [f] [1,4] oxazepin-3 (2H)-ones (5an) (Scheme 1) which can be further used in evaluation of the mechanisms of the physiological action of these compounds.

Additionally, we have also studied the transmission of substituent effects on IR thiocarbonyl stretching frequencies ν(Cdouble bondS) of 4-substituted phenyl-4,5-dihydrobenzo [f] [1,4] oxazepin-3 (2H)-thiones (6an) (Scheme 1). Substituent effects on Cdouble bondO [2], [3], [4], [6], [7], [8], [10], [11], CN [5], SO2 [4], [8] and N–H [8], [9] stretching frequencies have been studied previously. However, as far as we know, a substituent effect study on IR ν(Cdouble bondS) is not present in the literature.

We also performed density functional theory (DFT) calculations on compounds (5) and (6) to characterize their three-dimensional structures and to predict their carbonyl and thiocarbonyl stretching frequencies.

Section snippets

Experimental

Schiff bases (2) (Scheme 1) were synthesized according to the literature [18]. The bands for the azomethine groups of (2) were observed in the IR spectra at about 1620 cm−1. For the synthesis of compounds (36), literature methods [19], [20], [21] were applied with slight modifications.

Salicylaldehyde (1) was reacted with substituted aniline to afford 2-[(E)-(substituted phenylimino) methylphenols (2) which were reduced by NaBH4 to obtain 2-(substituted phenylamino) phenols (3). Compounds (3)

IR spectral study

The IR ν(Cdouble bondO) and ν(Cdouble bondS) of compounds (5an) and (6an), respectively are listed in Table 1. The low ν(Cdouble bondO) and ν(Cdouble bondS) were observed.

When strong electron donating groups were present in the phenyl ring. Contrarily, high frequencies were noted when strong electron attracting groups were present in the same ring (Fig. 1, Fig. 2). A similar trend in the absorption was also observed earlier [2], [3], [4], [6].

ν(Cdouble bondO) of compounds (5an) provide fair correlation with σ and a better correlation with σ

Conclusions

It has been shown that substituent effects from the aryl group are efficiently transmitted to the heterocyclic framework of compounds (5) and (6). This transmission has been characterized by fair correlations between the Hammett-type constants of the substituents and Cdouble bondO (or Cdouble bondS) stretching wave numbers of the seven-membered heterocyclic ring system.

Acknowledgement

The authors thank Kocaeli University Research Fund for financial support (Grant No. 2004/34).

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