Study on the solvent effects of benzamide in single solvents by FTIR

<strong>Study</strong> on the solvent effects of benzamide in single solvents
by FTIR
Feiying Zhang, Huali Song, Hui Zhang, Qing Liu *
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
Abstract:Infrared spectroscopy studies of benzamide (BA) in 18 different organic
solvents, both polar and nonpolar, were undertaken to investigate the solvent-solute
interactions. The frequencies of carbonyl stretching vibration (ν(C=O)) of BA were
correlated with the empirical parameters of the solvents such as the
Kirkwood-Bauer-Magat (KBM) equation, the solvent acceptor number (AN), Swain’s
parameters (Aj and Bj) and the linear solvation energy relationships (LSER). The
applicabilities of different solvent parameters were discussed and the solute-solvent
interactions in single solvents were studied in detail .The solvent-induced carbonyl
stretching vibration frequency shifts of BA showed the best correlation with the LSER
than the others.
Keywords: Carbonyl stretching vibration frequency; Benzamide; Linear solvation
energy relationships; Swain’s parameters; Solvent acceptor number; Kirkwood-Bauer
-Magat equation
* Corresponding author. Fax: 86-571-87951289; Tel: 86-571-87951289;
E-mail address: liuqing@zju.edu.cn
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1. Introduction
Infrared spectroscopy study provides an important tool for the qualitative study
of solvent-solute interactions [1-5]. A number of attempts to develop a quantitatively
accurate and physically meaningful explanation of solvent-induced vibration
frequency shifts of the solute have been also presented [6-14].
The first theoretical treatment of solvent-induced vibration frequency shifts was
given by KBM, which is shown in Eq.(1):
(ν o -ν s ) / ν o =Δν / ν o = C(ε-1)/(2ε+1) (1)
ν o is the vibrational frequency of a solute in the gas phase, ν s is the vibrational
frequency of a solute in the solvent, ε is the dielectric constant of solvent and C is a
constant depending upon the dimensions and electrical properties of the vibrating
solute dipole.
Because the ν o could not be obtained from the experiment, the Eq (1) can be
modified into a generalized form (shown in Eq (1’) ), where the B equal to C*ν o .
When the correlation between the ν s and (ε-1)/(2ε+1) is linear, the slope is B and the
intercept is ν o .
ν s = ν o - B (ε-1)/(2ε+1) (1’)
The solvent acceptor number (AN) is one of the empirical parameters of solvent
shown in Eq. (2), which was developed by Gutmann from the 31 P NMR chemical
shifts of triethylphosphane oxide to study the solvent effects [15–19], and is
reportedly a measure of the electrophilicity (or the Lewis acidity) of the solvent:
AN = [δ (solv) − δ (hexane) ] × 2.348 =Δδ × 2.348 (2)
where δ (solv) and δ (hexane) are the 31 P NMR chemical shifts of triethylphosphane oxide
in a solvent and in hexane, respectively. The solvent acceptor numbers (AN) have
been found to be useful in the prediction of 13 C chemical shifts and infrared
vibrational band shifts in different solvents for some simple molecules [20]. The
relation suitable to infrared spectroscopy is shown in Eq. (3):
ν =ν 0 + K∗ AN (3)
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where ν 0 is the infrared vibrational frequency of a solute in hexane and K is the
sensitivity of infrared vibrating frequency (ν) to the solvent acceptor numbers (AN).
Swain proposed a two-parameter treatment of solvent effects [22]. It is based on
a computer calculation involving 1080 data sets for 61 solvents and 77
solvent-sensitive reactions and physicochemical properties. According to these
calculations, all solvent effects can be rationalized in terms of two complementary
solvent property scales, i.e. A j , measuring the solvent’s anion-solvating tendency or
acidity, and B j , measuring the solvent’s cation-solvating tendency or basicity [21,22],
both combined in Eq.(4):
ν = ν 0 +a i ∗A j +b i ∗ Bj (4)
ν is the vibration frequency of solute in solvent j. ν 0 represents the predicated value
for a reference solvent (n-heptane) for which A j = B j = 0.00. a i and b i represent the
sensitivity of solute to a solvent change.
The linear solvation energy relationships (LSER) is a multiparameter (that is, the
Kamlet-Taft solvent parameters α, β, π* and δ ) treatment used to describe the solvent
effects [21,23-25]. The model equation of LSER applied in infrared spectroscopy is
shown in Eq.(5):
ν = ν 0 + (sπ* + dδ) + aα + bβ (5)
ν is the vibration frequency of solute (such as ν(C=O)) in a pure solvent and ν 0 is the
regression value of the ν(C=O) in cyclohexane as a reference solvent. π* is an index
of solvent dipolarity/polarizability. δ is a discontinuous polarizability correction term
for poly-chlorinesubstituted aliphatics and aromatic solvents. α is a measure of the
solvent hydrogen-bond donor acidity. β is a measure of the solvent hydrogen-bond
acceptor basicity. The regression coefficients s, d, a, and b in Eq. (5) can provide
quantitative measures of the relative contribution of the indicated parameters.
Benzamide (BA), a useful medicine intermediate, is widely used for
antipsychotic drugs synthesis. In the present work, the infrared spectroscopy is
applied to research the solvent effects in single solvents to probe into the mechanism
of the solute-solvent interactions and make a conclusion on the practical application
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of different solvent parameters.
2. Experimental details
All solvents used were of analytical purity. BA was dried and weighed before
being added into the solvent. The concentrations of the solution ranged from 1.0×10 -3
to 5.0× 10 -3 mol/L.
The spectra of the solutions were measured using a Nicolet Nexus 670 FTIR
spectrometer with a Ge/KBr beamsplitter and a DTGS detector at room temperature
(21±2 °C) with the spectrum region 4000- 400cm -1 . For all spectra, 40 scans
recorded at 1cm -1 resolution were averaged. Solution spectra were measured using 0.1
mm path length NaCl cells as a sample. The solvent spectra were scanned at the same
conditions as a background. The Nicolet OMNIC software, version 7.3, was used for
all data manipulation. The data files were transferred to a computer for analysis using
a digital curve-fitting program (OriginPro 7.5).
3.Results and discussion
The ν(C=O) of BA in 18 different pure solvents and the solvent parameters are
presented in Table 1. The C=O stretching vibration band of BA appears in the region
of 1675-1695 cm -1 .
Fig. 1 is a plot of ν(C=O) of BA versus (ε-1)/(2ε+1) of the KBM equation of the
solvent. It is evident that there is no linear correlation between ν(C=O) and the KBM
parameters. The carbonyl stretching vibrational frequency shift of the benzamide
molecule in the solution is a complex function of solvent properties. The frequency
shifts depend not only on solvent dielectric constants, but also on other solvent-solute
interactions such as the hydrogen-bonding. The KBM equation only takes account of
the solvent dielectric constants. Therefore, the correlation between ν(C=O) and the
KBM parameters is poor. It indicates that the KBM relationship which only takes into
account the dielectric constants is unsuitable for the correlative analysis of solvent
effects in this paper.
Fig. 2 is a plot of ν(C=O) of BA versus the solvent acceptor number (AN) and
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shows two different lines. It means that there are two kinds of solvent-induced
carbonyl bands of BA. The first one (band A) is the carbonyl absorption of BA in the
non-alcoholic solvents, and the second (band B) is the one in the alcoholic solvents.
The linear correlation analysis results and correlation coefficient are shown in Eq. (6)
and (7), respectively:
Band A: ν A = – 0.65AN + 1694.85 (6)
R 2 = 0.826 SD =2.167 cm -1
Band B: ν B = – 0.10AN + 1679.33 (7)
R 2 = 0.763 SD =0.304 cm -1--
The negative slopes of the two lines demonstrate that the ν(C=O) of BA are
shifted to lower wavenumbers as the Lewis acidity of the solvent increases. Because
the AN is an experiential parameter, which expresses the electroaffinity of solvent as
electron pair acceptor. With the ability of accepting electron pair of the solvent
enhances, the value of the solvent AN increases and the intensity of the interaction
between the solute as the electron pair donor and the solvent strengthens, which lead
to the electron density of C=O decreases and the frequency of carbonyl stretching
vibration shifts to a lower one.
From the Fig. 2, it can be observed that the slope of the correlation line of band
B is smaller than that of band A. It means the shift of ν(C=O) in alcoholic solvents is
less sensitive to the solvent AN. Because the ν(C=O) in alcohol is affected by the
strength of intermolecular hydrogen bond between the C=O group and alcoholic OH
proton, which depends upon the polarity of the solute molecules, the acidity of the
alcoholic OH proton and the steric factors. In alcoholic solvent, the alcohol molecule
is easy to associate. The associated alcohol molecule is like a large molecule. The
steric factor of the different associated alcohol molecule is alike, and the acidity of the
different associated alcohol molecules is also similar. As a result, the interaction
between the C=O of BA and the self-associated alcohols is similar, which makes the
ν(C=O) in alcoholic solvents is less sensitive to the solvent parameter AN.
The Swain’s equations for the carbonyl stretching vibration band of BA is given
in Eq.(8):
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ν (C=O) = 1696.74 – 28.87A(j) – 5.42B(j) (8)
R 2 = 0.912 SD = 2.140 cm -1
The solvent effects are divided into two species by Swain. One is the solvent’s
anion-solvating tendency (acidity), and the other is the solvent’s cation-solvating
tendency (basicity). It means only the specific solute-solvent interactions are
considered.
The linear correlation of ν(C=O) for BA with Swain’s solvent parameters is
better than the one with AN. It is probable that both the solvent Lewis acidity and the
solvent Lewis basicity are considered in Swain’s equation, but the solvent acceptor
number only takes the solvent Lewis acidity into account.
The negative signs of the regression coefficients of Aj and Bj represent the same
influences on the C=O of benzamide: the hydrogen-bond donor acidity of solvents
and the hydrogen-bond acceptor basicity of solvents lead to the C=O band red shift.
The ratio of the regression coefficients of Aj and Bj is equal to 5.33, which means that
the red shift of the C=O band induced by the solvent acidity is larger than the one
induced by the solvent basicity.
LSER developed by Kamlet, Taft and co-workers has been used to better
understand and identify the different solvent–solute and solvent–solvent
intermolecular interactions. LSER is a powerful tool for the study of the principal
intermolecular interactions that control a specific physicochemical process in solution,
such as intermolecular electron transfer associated with a free-energy change. LSER
assumes that the different solvent/solute interactions are additive and can be
categorized in two groups: the exoergic and endoergic interactions. The exoergic
interactions have their origins in attractive solute/solvent interactions and can be
quantified by the solvatochromic parameters π*, α and β. These parameters can be
classified in non-specific (π*) and specific (α and β). The π* parameter measures the
exoergic effects of dipole/dipole and dipole/induced dipole interactions between the
solute and the solvent molecules. A discontinuous polarizability correction term, δ
must be added to π* parameter when polichlorinesubstituted aliphatics and aromatic
solvents are used. The solvachromic parameter a is a quantitative empirical measure
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of the ability of a bulk solvent to act as hydrogen-bond donor towards a solute. By
contrast, the empirical parameter β measures the ability of a bulk solvent to act as a
hydrogen-bond acceptor or electron-pair donor towards a given standard solute.
The LSER regression coefficients using the data for 18 solvents in Table 1 are
calculated and the linear correlation analysis results and correlation coefficient are
shown in Eq.(9):
ν (C=O) = 1701.08 – 16.22π* – 2.91δ – 12.72α – 8.40β (9)
R 2 = 0.928 SD = 2.098 cm -1
Then a linear correlation between the C=O stretching frequencies of BA
calculated from the Eq.(9) and the ones observed from the experiment is illustrated in
Fig. 3 with the correlation factor (R 2 ) of 0.919 and the standard deviation (SD) of
1.991cm -1 . In the LSER equations, there are not only the specific interaction
parameters (measured by α, β) but also the non-specific interaction parameters
(measured by π*). The regression coefficients s, d, a, and b in equation measure the
relative susceptibilities of the solvent-dependent solute property ν to the indicated
solvent parameters [21]. The regression coefficients of π* and α are close (the former
equals 16.22 and the later is 12.72), which means the susceptibility of C=O band of
BA to the dipolarity/polarizability of solvents is similar to the hydrogen-bond donor
acidity of solvents. Consequently a complete description of all solute-solvent
interactions must include both non-specific and specific effects. On the other hand,
the two values are negative. It indicates both of them could induce the carbonyl band
of BA red shift. The regression coefficient of α is bigger than the regression
coefficient of β that means the C=O of benzamide is more susceptible to the
hydrogen-bond donor acidity of solvents than the solvent hydrogen-bond acceptor
basicity. The regression coefficient of α and β are negative, which represent the same
influences on the C=O of benzamide: the hydrogen-bond donor acidity of solvents
(measured by α) and the hydrogen-bond acceptor basicity of solvents (measured by β)
lead to the C=O band red shift.
4. Conclusions
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No linear relationship exists between the ν(C=O) of BA and KBM solvent
parameters. It indicates that the KBM relationship which only takes into account the
dielectric constants is unsuitable for the correlative analysis of solvent effects in this
paper.
Comparing the correlation of the ν(C=O) with the solvent parameter KBM and
the solvent parameter AN, the latter is better than the former. The similarity of the
electrophilic ability for the associated alcohols makes the ν(C=O) of BA in different
alcohols be less sensitive to solvent AN.
The linear correlation of the ν(C=O) with Swain’s parameters is better than the
one with AN. But the correlation of the ν(C=O) with the LSER parameters of the
solvents is the best. It indicates that the special and non-special solute-solvent
interactions should be considered together to describe the solvent effects.
Acknowledge
The Analysis and Measurement Foundation of Zhejiang Province, P.R. China
supported this work(2007F70069).
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Fig. 1. A plot of ν(C=O) vs KBM parameter (ε-1)/(2ε+1)
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Fig. 2. A plot of ν(C=O) vs the solvent acceptor number
Fig.3. The carbonyl frequencies of benzamide calculated from the LSER (Eq.(9)) versus the
carbonyl frequencies observed from the experiment
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