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Solvent effects on infrared spectra of 2-acetylthiophene in organic solvents
Spectrochimica Acta Part A 59 (2003) 471 /475 www.elsevier.com/locate/saa
Solvent effects on infrared spectra of 2-acetylthiophene in organic solvents Qing Liu , Xiaomin Xu, Wenqiang Sang Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 25 March 2002; accepted 12 April 2002
Abstract Infrared spectroscopy studies of 2-acetylthiophene (ACTH) in 18 different organic solvents, both polar and nonpolar, were undertaken to investigate the solvent /solute interactions. The frequencies of carbonyl stretching vibration y(C/O) of ACTH were correlated with the properties such as the solvent acceptor number (AN) and the linear solvation energy relationships (LSER). The solvent-induced stretching vibration frequency shifts showed a better correlation with the LSER than the AN. A six-membered ring-like hydrogen bonding structure was presented and the solvent effects of ACTH in alcohol solvents were investigated in detail. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbonyl stretching vibration frequency; 2-Acetylthiophene; Linear solvation energy relationships; Hydrogen-bond; Sixmembered ring-like structure
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 stretching vibration frequency shifts have been also presented [6 /14]. The solvent acceptor number (AN) is one of the empirical parameters which was developed by Gutmann from the 31P-NMR chemical shifts of triethylphosphane oxide to study
Corresponding author. Tel.: /86-571-87951289; fax: /86571-87951995 E-mail address: [email protected] (Q. Liu).
the solvent effects [15], and is reportedly a measure of the electrophilicity (or Lewis acidity) of the solvent. The linear solvation energy relationships (LSER) is a multiparameter (that is, the Kamlet /Taft solvent parameters a , b, p and d ) treatment used to describe the solvent effects [15 / 18]. The model equation of LSER applied in infrared spectroscopy is shown in Eq. (1) y y0 (sp dd)aabb
(1)
y is the vibration frequency of solute (such as y (C /O)) in pure solvent and y0 is the regression value of they (C /O) in cyclohexane as a reference solvent. p is an index of solvent dipolarity/ polarizability. d is a discontinuous polarizability correction term for poly-chlorinesubstituted ali-
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 2 1 5 - 9
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Q. Liu et al. / Spectrochimica Acta Part A 59 (2003) 471 /475
phatics and aromatic solvents. a is a measure of the solvent hydrogen-bond donor acidity. b is a measure of the solvent hydrogen-bond acceptor basicity. The regression coefficients s, d , a, and b in Eq. (1) measure the relative susceptibilities of the solvent-dependent vibration frequency of the solute to indicate solvent parameters. 2-Acetylthiophene (ACTH) is a medicine intermediate and contains an acetyl group conjugated with the thiophene ring. It is selected as a model compound in the present paper. Some interesting details of the interactions between alcohols and ACTH have been found in the following discussion.
2. Experimental Infrared spectra were recorded on a Nicolet Nexus 670 FTIR spectrometer with a Ge/KBr beamsplitter and a DTGS detector. For all spectra, 64 scans recorded at 1 cm 1 resolution were averaged. Solution spectra were measured using 1.0-mm pathlength NaCl cells. The spectra of pure solvents were recorded under the same conditions and were stored on the computer. All solvents were either of analytical or spectroscopic purity and were distilled prior to use. The concentrations of the solute in the pure solvents were between 1.0 /10 5 and 1.5 /10 5 mol l 1. In the case of strongly overlapping bands in the solution spectra, the interactive spectral subtraction was performed. The Nicolet Omnic software Version 5.2 was used for all data manipulation. The data files were transferred to a computer for analysis using a digital curve-fitting program.
3. Results and discussion The y (C /O) of ACTH in 18 different pure solvents and the solvent parameters are presented in Table 1. The C /O stretching vibration band of ACTH appears in the region of 1630/1670 cm 1. Two remarkable bands are observed for the y (C / O) of ACTH in alcohol solvents. It means that two species of C /O for ACTH coexist in alcohols. The frequencies of the lower frequency band are listed
in Table 1(band A) together with the C /O band in aprotic solvents, and the frequencies of the higher one are independently listed in Table 1(band B). Successful correlations are found between the y (C /O) of ACTH and the solvent AN for band A (Fig. 1). The result is shown in Eq. (2) with a correlation coefficient of 0.975. yA 0:51AN1675:1
(2)
The result can be considered as the interactions of electrons on the C /O group of ACTH with the solvent as a Lewis acid [19]. The negative slope demonstrates that the y (C /O) of ACTH is shifted to lower frequencies as the Lewis acidity of the solvent increases. It means that the decreases of electron density by solvent effects lead to weakening of the C /O bond. The remarkable solvent effect is observed in methanol, in which the y (C/ O) of ACTH is 1655.23 cm 1. It is lower than that in n -hexane (1678.78 cm 1) (Dv /23.55 cm 1) owing to stronger intermolecular hydrogen-bond between the C /O of ACTH and the hydroxyl (OH) of alcohols. The LSER equation for band A is presented in Eq. (3) with a correlation coefficient of 0.995 and estimated standard deviation of 0.84 cm 1. yA (1677:7290:55) [(12:0890:95)p (1:3590:50)d] (14:2490:91)a (3:3591:02)b
(3)
Koppel and Palm have been argued that a complete description of all solvent /solute interactions must include both non-specific and specific effects [20,21]. The former should be subdivided into solvent dipolarity/polarizability (measured by p in LSER) and the discontinuous polarizability correction term (d ). The latter is solvent Lewis acidity and basicity (measured by a andb , respectively). Eq. (3) shows that the solvent dipolarity/ polarizability (p ) plays an important role in interactions between aprotic solvent and the C / O of ACTH since a /0 and (s/b) /3.6 (s /b is the ratio of the regression coefficients of p and b in Eq. (3)). In protic solvents, the solvent acidity (a ) is the most sensitive factor since [a /b ] /4.3 ([a /b ] is the ratio of the regression coefficients of a andb in Eq. (3)). Therefore, the frequencies of
Q. Liu et al. / Spectrochimica Acta Part A 59 (2003) 471 /475
473
Table 1 The y (C /O) of ACTH in different pure solvents and solvent parameters Solvent
y (C/O)a cm 1 Band A
n -Hexane Cyclohexane Diethyl ether Toluene Tetrahydrofuran Benzene Tetrachloromethane 1,4-Dixone Acetonitrile Dichloromethane Chloroform tert -Butanol 2-Butanol 2-Propanol n -Butanol n -Propanol Ethanol Methanol a b c d e f
1678.78 1677.52 1673.69 1670.28 1668.72 1668.63 1672.32 1668.21 1665.28 1664.05 1661.80 1660.59 1658.60 1657.81 1656.98 1656.81 1656.05 1655.23
ANb
bc
ac
p c
dd
yA(C /O)e
Dv f
0 0 3.9 4.5 8 8.2 8.6 10.8 18.9 20.4 23.1 27.1 32.0 33.6 36.8 37.3 37.9 41.5
0 0 0.47 0.11 0.55 0.10 0 0.37 0.31 0 0 1.01 0.95 0.95 0.88 / 0.77 0.62
0 0 0 0 0 0 0 0 0.19 0.3 0.44 0.68 0.76 0.76 0.79 0.78 0.83 0.93
/0.08 0 0.27 0.54 0.58 0.59 0.28 0.55 0.75 0.82 0.58 0.41 0.48 0.48 0.47 0.52 0.54 0.60
0 0 0 0.7 0 0.8 1.7 0 0 0.5 1.5 0 0 0 0 0 0 0
1678.69 1677.72 1672.88 1669.89 1668.87 1669.18 1672.04 1669.84 1664.92 1662.87 1662.42 1659.7 1657.92 1657.92 1657.84 / 1656.8 1655.15
/0.09 0.2 /0.81 /0.39 0.15 0.55 /0.28 1.63 /0.36 /1.18 0.62 /0.89 /0.68 0.11 0.86 / 0.75 /0.08
Band B
1673.68 1671.85 1672.35 1671.97 1671.88 1671.44 1670.04
The frequencies of y (C /O) from experiment. Acceptor number [15]. Kamlet /Taft solvent parameters [15]. See Ref. [18]. The y (C/O) of band A calculated by Eq. (3). Dv /yA(C/O)cal /yA(C/O)exp.
Eq. (4) with a correlation coefficient of 0.988. yB 0:20AN1678:8
Fig. 1. A plot of y (C/O) of ACTH vs. the solvent AN. (Band A: I; Band B: ').
y (C /O) are much lower in alcohols and chloroform owing to hydrogen bonding effects. The correlation between the frequencies of y (C / O) for band B and AN of solvents is presented in
(4)
The datum of frequency in n -hexane is added to this equation because n -hexane is a reference solvent, its AN equal to zero. The change of y (C /O) is similar to that in band A, a red shift of the frequency is observed as AN increases. The good correlation indicates that the C /O belong to this species also shows some Lewis basicity. However, the frequency of y (C /O) for band B in methanol (1670.04 cm 1) is only 8.7 cm 1 lower than that in n -hexane and the slope in Eq. (4) (0.20) is less than that in Eq. (2) (0.51). The LSER equation for band B is presented in Eq. (5) with a correlation coefficient of 0.991 and estimated standard deviation of 0.25 cm 1. yB (1694:6490:25) (5:4290:44)p (19:2291:60)a (5:6390:20)b
(5)
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The role of a is more important than that of b according to [a/b] /3.4. However, the contribution of b somewhat increases compared with that of band A because the value of [a/b ] in Eq. (3) is bigger than that in Eq. (5). The correlations that the y (C /O) of band B versus a and b are shown in Fig. 2 and Fig. 3, respectively. Successful correlations for both parameters are found. The result equations are yB 13:82a1682:8
(6)
with a correlation coefficient of 0.976 as to a and yB 7:57b1665:4
Fig. 3. A plot of y (C /O) of ACTH for band B vs. b (Kamlet / Taft solvent parameter) in pure alcohol solvents.
(7)
with a correlation coefficient of 0.922 as to b. As mentioned above, solvent parameters a and b are the measure of solvent hydrogen-bond donor acidity and acceptor basicity, respectively. The good correlations between y (C /O) and each of the parameters indicate that alcohols behave not only Lewis acidity (by hydroxyl hydrogen) but also Lewis basicity (by hydroxyl oxygen) when involving the species of band B. Accordingly, ACTH would have the same case. A reasonable structure is shown in Scheme 1. It implies the presence of two hydrogen bonds, one is between the oxygen atom of alcohol and the methyl hydrogen atom of ACTH (O/ /H/C) and the other is between the carbonyl oxygen and the alcohol hydrogen atom (C/O/ /H/O). The conjugation of thiophene ring and acetyl makes it possible for one of the acetyl hydrogen atoms to interact with the hydroxyl oxygen atom in some degree.
Scheme 1.
Han and Kim [22] conducted a matrix isolation study of acetone and methanol in solid argon. They suggested that H/O/ /H /C and C /O/ /H / O hydrogen bonds coexist in the system. The theoretical calculations showed a planar six-membered ring-like structure possessing the above two kinds of hydrogen bonds. The C /O/ /H /O hydrogen-bond of this species is relatively weaker than that of band A for the existence of ring-like structure. According to the above discussions, a similar structure (Scheme 1) is assumed to exist in the studied system in room temperature.
4. Conclusions
Fig. 2. A plot of y (C /O) of ACTH for band B vs. a (Kamlet / Taft solvent parameter) in pure alcohol solvents.
A good correlation between the y (C /O) of band A and the solvent AN is observed. The correlation between the y (C /O) of this band and the LSER parameters of the solvents is also excellent. The LSER offers a quantitatively accurate and physically meaningful explanation of solvent-induced stretching frequency shifts. This fact allows the prediction of the frequency of the C /O stretching band of ACTH in other solvents if their LSER parameters are obtained (as listed in Table 1, the difference between the y (C /O) of experiment and
Q. Liu et al. / Spectrochimica Acta Part A 59 (2003) 471 /475
calculation for band A are less than 1 cm 1 except those in dichloromethane and 1,4-dixone). A second band (band B) is observed in alcohols. The y (C /O) of this band for ACTH also correlate well with the solvent AN, LSER, and Kamlet / Taft solvent parameters a , act as a Lewis acid and b , act as a Lewis base, respectively. The structure corresponding to band B is presented, which implies a six-membered ring-like structure with coexistence of the hydrogen bonds of H /O/ / H /C and C /O/ /H /O between ACTH and alcohols.
References [1] R.P. Sijbesma, A.P.M. Kentgens, E.T.G. Lutz, J.H. Van der Maas, R.J.M. Nolte, J. Am. Chem. Soc. 115 (1993) 8999. [2] P. Bruni, C. Conti, R. Galeazzi, J. Mol. Struct. 480 /481 (1999) 379. [3] A. Wener, (Ed.), Structrure and Dynamics of Weakly Bond Molecular Complexed; NATO Advanced Scientific Institute Seris C, 1987, 212. [4] A. Perjessy, J.B.F.N. Engberts, Monatsh. Chem. 126 (1995) 871.
475
[5] J.-N. Cha, B.-S. Cheong, H.-G. Cho, J. Mol. Struct. 570 (2001) 97. [6] A.A. Stolov, J. Mol. Struct. 480 /481 (1999) 499. [7] B. Hernandez, J. Mol. Struct. 565 /566 (2001) 259. [8] A. Zimniak, J. Mol. Struct. 433 (1998) 115. [9] I. Bratu, Spectrochim. Acta 54A (1998) 501. [10] D.K. Cha, A.A. Kloss, A.C. Tikanen, W.R. Fawcett, Phys. Chem. Chem. Phys. 1 (1999) 4785. [11] R. Streck, Spectrochim. Acta 55 (1999) 1049. [12] M.M. Wohar, Spectrachimica Acta 54A (1998) 999. [13] R.A. Nyquist, App. Spectroscopy 43 (1989) 1053. [14] R. Navarro, J. Mol. Struct. 348 (1995) 253. [15] C. Rcichardt, Solvents and Solvent Effects in Organic Chemistry, second ed. rev. and enl. ed., Verlag Chemie Weinheim, New York, 1988. [16] M.J. Kamlet, J.L. Abbound, R.W. Taft, J. Am. Chem. Soc. 99 (1977) 6027. [17] J.L. Abbound, M.J. Kamlet, R.W. Taft, Progr. Phys. Org. Chem. 13 (1981) 485. [18] Q. Liu, W.Q. Sang, X.M. Xu, J. Mol. Struct. 608 (2002) 253. [19] M.J. Kamlet, J.L. Abound, R.W. Taft, J. Org. Chem. 48 (1983) 2877. [20] R.A. Nyquist, Appl. Spectrosc. 44 (1990) 433. [21] I.A. Koppel, V.A. Palm, Organic Reactivity (Tartu) 8 (1971) 296. [22] S.W. Han, K. Kim, J. Phys. Chem. 100 (1996) 17124.
Solvent effects on infrared spectra of 2-acetylthiophene in organic solvents Qing Liu , Xiaomin Xu, Wenqiang Sang Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 25 March 2002; accepted 12 April 2002
Abstract Infrared spectroscopy studies of 2-acetylthiophene (ACTH) in 18 different organic solvents, both polar and nonpolar, were undertaken to investigate the solvent /solute interactions. The frequencies of carbonyl stretching vibration y(C/O) of ACTH were correlated with the properties such as the solvent acceptor number (AN) and the linear solvation energy relationships (LSER). The solvent-induced stretching vibration frequency shifts showed a better correlation with the LSER than the AN. A six-membered ring-like hydrogen bonding structure was presented and the solvent effects of ACTH in alcohol solvents were investigated in detail. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbonyl stretching vibration frequency; 2-Acetylthiophene; Linear solvation energy relationships; Hydrogen-bond; Sixmembered ring-like structure
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 stretching vibration frequency shifts have been also presented [6 /14]. The solvent acceptor number (AN) is one of the empirical parameters which was developed by Gutmann from the 31P-NMR chemical shifts of triethylphosphane oxide to study
Corresponding author. Tel.: /86-571-87951289; fax: /86571-87951995 E-mail address: [email protected] (Q. Liu).
the solvent effects [15], and is reportedly a measure of the electrophilicity (or Lewis acidity) of the solvent. The linear solvation energy relationships (LSER) is a multiparameter (that is, the Kamlet /Taft solvent parameters a , b, p and d ) treatment used to describe the solvent effects [15 / 18]. The model equation of LSER applied in infrared spectroscopy is shown in Eq. (1) y y0 (sp dd)aabb
(1)
y is the vibration frequency of solute (such as y (C /O)) in pure solvent and y0 is the regression value of they (C /O) in cyclohexane as a reference solvent. p is an index of solvent dipolarity/ polarizability. d is a discontinuous polarizability correction term for poly-chlorinesubstituted ali-
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 2 1 5 - 9
472
Q. Liu et al. / Spectrochimica Acta Part A 59 (2003) 471 /475
phatics and aromatic solvents. a is a measure of the solvent hydrogen-bond donor acidity. b is a measure of the solvent hydrogen-bond acceptor basicity. The regression coefficients s, d , a, and b in Eq. (1) measure the relative susceptibilities of the solvent-dependent vibration frequency of the solute to indicate solvent parameters. 2-Acetylthiophene (ACTH) is a medicine intermediate and contains an acetyl group conjugated with the thiophene ring. It is selected as a model compound in the present paper. Some interesting details of the interactions between alcohols and ACTH have been found in the following discussion.
2. Experimental Infrared spectra were recorded on a Nicolet Nexus 670 FTIR spectrometer with a Ge/KBr beamsplitter and a DTGS detector. For all spectra, 64 scans recorded at 1 cm 1 resolution were averaged. Solution spectra were measured using 1.0-mm pathlength NaCl cells. The spectra of pure solvents were recorded under the same conditions and were stored on the computer. All solvents were either of analytical or spectroscopic purity and were distilled prior to use. The concentrations of the solute in the pure solvents were between 1.0 /10 5 and 1.5 /10 5 mol l 1. In the case of strongly overlapping bands in the solution spectra, the interactive spectral subtraction was performed. The Nicolet Omnic software Version 5.2 was used for all data manipulation. The data files were transferred to a computer for analysis using a digital curve-fitting program.
3. Results and discussion The y (C /O) of ACTH in 18 different pure solvents and the solvent parameters are presented in Table 1. The C /O stretching vibration band of ACTH appears in the region of 1630/1670 cm 1. Two remarkable bands are observed for the y (C / O) of ACTH in alcohol solvents. It means that two species of C /O for ACTH coexist in alcohols. The frequencies of the lower frequency band are listed
in Table 1(band A) together with the C /O band in aprotic solvents, and the frequencies of the higher one are independently listed in Table 1(band B). Successful correlations are found between the y (C /O) of ACTH and the solvent AN for band A (Fig. 1). The result is shown in Eq. (2) with a correlation coefficient of 0.975. yA 0:51AN1675:1
(2)
The result can be considered as the interactions of electrons on the C /O group of ACTH with the solvent as a Lewis acid [19]. The negative slope demonstrates that the y (C /O) of ACTH is shifted to lower frequencies as the Lewis acidity of the solvent increases. It means that the decreases of electron density by solvent effects lead to weakening of the C /O bond. The remarkable solvent effect is observed in methanol, in which the y (C/ O) of ACTH is 1655.23 cm 1. It is lower than that in n -hexane (1678.78 cm 1) (Dv /23.55 cm 1) owing to stronger intermolecular hydrogen-bond between the C /O of ACTH and the hydroxyl (OH) of alcohols. The LSER equation for band A is presented in Eq. (3) with a correlation coefficient of 0.995 and estimated standard deviation of 0.84 cm 1. yA (1677:7290:55) [(12:0890:95)p (1:3590:50)d] (14:2490:91)a (3:3591:02)b
(3)
Koppel and Palm have been argued that a complete description of all solvent /solute interactions must include both non-specific and specific effects [20,21]. The former should be subdivided into solvent dipolarity/polarizability (measured by p in LSER) and the discontinuous polarizability correction term (d ). The latter is solvent Lewis acidity and basicity (measured by a andb , respectively). Eq. (3) shows that the solvent dipolarity/ polarizability (p ) plays an important role in interactions between aprotic solvent and the C / O of ACTH since a /0 and (s/b) /3.6 (s /b is the ratio of the regression coefficients of p and b in Eq. (3)). In protic solvents, the solvent acidity (a ) is the most sensitive factor since [a /b ] /4.3 ([a /b ] is the ratio of the regression coefficients of a andb in Eq. (3)). Therefore, the frequencies of
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473
Table 1 The y (C /O) of ACTH in different pure solvents and solvent parameters Solvent
y (C/O)a cm 1 Band A
n -Hexane Cyclohexane Diethyl ether Toluene Tetrahydrofuran Benzene Tetrachloromethane 1,4-Dixone Acetonitrile Dichloromethane Chloroform tert -Butanol 2-Butanol 2-Propanol n -Butanol n -Propanol Ethanol Methanol a b c d e f
1678.78 1677.52 1673.69 1670.28 1668.72 1668.63 1672.32 1668.21 1665.28 1664.05 1661.80 1660.59 1658.60 1657.81 1656.98 1656.81 1656.05 1655.23
ANb
bc
ac
p c
dd
yA(C /O)e
Dv f
0 0 3.9 4.5 8 8.2 8.6 10.8 18.9 20.4 23.1 27.1 32.0 33.6 36.8 37.3 37.9 41.5
0 0 0.47 0.11 0.55 0.10 0 0.37 0.31 0 0 1.01 0.95 0.95 0.88 / 0.77 0.62
0 0 0 0 0 0 0 0 0.19 0.3 0.44 0.68 0.76 0.76 0.79 0.78 0.83 0.93
/0.08 0 0.27 0.54 0.58 0.59 0.28 0.55 0.75 0.82 0.58 0.41 0.48 0.48 0.47 0.52 0.54 0.60
0 0 0 0.7 0 0.8 1.7 0 0 0.5 1.5 0 0 0 0 0 0 0
1678.69 1677.72 1672.88 1669.89 1668.87 1669.18 1672.04 1669.84 1664.92 1662.87 1662.42 1659.7 1657.92 1657.92 1657.84 / 1656.8 1655.15
/0.09 0.2 /0.81 /0.39 0.15 0.55 /0.28 1.63 /0.36 /1.18 0.62 /0.89 /0.68 0.11 0.86 / 0.75 /0.08
Band B
1673.68 1671.85 1672.35 1671.97 1671.88 1671.44 1670.04
The frequencies of y (C /O) from experiment. Acceptor number [15]. Kamlet /Taft solvent parameters [15]. See Ref. [18]. The y (C/O) of band A calculated by Eq. (3). Dv /yA(C/O)cal /yA(C/O)exp.
Eq. (4) with a correlation coefficient of 0.988. yB 0:20AN1678:8
Fig. 1. A plot of y (C/O) of ACTH vs. the solvent AN. (Band A: I; Band B: ').
y (C /O) are much lower in alcohols and chloroform owing to hydrogen bonding effects. The correlation between the frequencies of y (C / O) for band B and AN of solvents is presented in
(4)
The datum of frequency in n -hexane is added to this equation because n -hexane is a reference solvent, its AN equal to zero. The change of y (C /O) is similar to that in band A, a red shift of the frequency is observed as AN increases. The good correlation indicates that the C /O belong to this species also shows some Lewis basicity. However, the frequency of y (C /O) for band B in methanol (1670.04 cm 1) is only 8.7 cm 1 lower than that in n -hexane and the slope in Eq. (4) (0.20) is less than that in Eq. (2) (0.51). The LSER equation for band B is presented in Eq. (5) with a correlation coefficient of 0.991 and estimated standard deviation of 0.25 cm 1. yB (1694:6490:25) (5:4290:44)p (19:2291:60)a (5:6390:20)b
(5)
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The role of a is more important than that of b according to [a/b] /3.4. However, the contribution of b somewhat increases compared with that of band A because the value of [a/b ] in Eq. (3) is bigger than that in Eq. (5). The correlations that the y (C /O) of band B versus a and b are shown in Fig. 2 and Fig. 3, respectively. Successful correlations for both parameters are found. The result equations are yB 13:82a1682:8
(6)
with a correlation coefficient of 0.976 as to a and yB 7:57b1665:4
Fig. 3. A plot of y (C /O) of ACTH for band B vs. b (Kamlet / Taft solvent parameter) in pure alcohol solvents.
(7)
with a correlation coefficient of 0.922 as to b. As mentioned above, solvent parameters a and b are the measure of solvent hydrogen-bond donor acidity and acceptor basicity, respectively. The good correlations between y (C /O) and each of the parameters indicate that alcohols behave not only Lewis acidity (by hydroxyl hydrogen) but also Lewis basicity (by hydroxyl oxygen) when involving the species of band B. Accordingly, ACTH would have the same case. A reasonable structure is shown in Scheme 1. It implies the presence of two hydrogen bonds, one is between the oxygen atom of alcohol and the methyl hydrogen atom of ACTH (O/ /H/C) and the other is between the carbonyl oxygen and the alcohol hydrogen atom (C/O/ /H/O). The conjugation of thiophene ring and acetyl makes it possible for one of the acetyl hydrogen atoms to interact with the hydroxyl oxygen atom in some degree.
Scheme 1.
Han and Kim [22] conducted a matrix isolation study of acetone and methanol in solid argon. They suggested that H/O/ /H /C and C /O/ /H / O hydrogen bonds coexist in the system. The theoretical calculations showed a planar six-membered ring-like structure possessing the above two kinds of hydrogen bonds. The C /O/ /H /O hydrogen-bond of this species is relatively weaker than that of band A for the existence of ring-like structure. According to the above discussions, a similar structure (Scheme 1) is assumed to exist in the studied system in room temperature.
4. Conclusions
Fig. 2. A plot of y (C /O) of ACTH for band B vs. a (Kamlet / Taft solvent parameter) in pure alcohol solvents.
A good correlation between the y (C /O) of band A and the solvent AN is observed. The correlation between the y (C /O) of this band and the LSER parameters of the solvents is also excellent. The LSER offers a quantitatively accurate and physically meaningful explanation of solvent-induced stretching frequency shifts. This fact allows the prediction of the frequency of the C /O stretching band of ACTH in other solvents if their LSER parameters are obtained (as listed in Table 1, the difference between the y (C /O) of experiment and
Q. Liu et al. / Spectrochimica Acta Part A 59 (2003) 471 /475
calculation for band A are less than 1 cm 1 except those in dichloromethane and 1,4-dixone). A second band (band B) is observed in alcohols. The y (C /O) of this band for ACTH also correlate well with the solvent AN, LSER, and Kamlet / Taft solvent parameters a , act as a Lewis acid and b , act as a Lewis base, respectively. The structure corresponding to band B is presented, which implies a six-membered ring-like structure with coexistence of the hydrogen bonds of H /O/ / H /C and C /O/ /H /O between ACTH and alcohols.
References [1] R.P. Sijbesma, A.P.M. Kentgens, E.T.G. Lutz, J.H. Van der Maas, R.J.M. Nolte, J. Am. Chem. Soc. 115 (1993) 8999. [2] P. Bruni, C. Conti, R. Galeazzi, J. Mol. Struct. 480 /481 (1999) 379. [3] A. Wener, (Ed.), Structrure and Dynamics of Weakly Bond Molecular Complexed; NATO Advanced Scientific Institute Seris C, 1987, 212. [4] A. Perjessy, J.B.F.N. Engberts, Monatsh. Chem. 126 (1995) 871.
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