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Solvent effects on the infrared spectra of β-alkoxyvinyl methyl ketones
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Spectrochimica Acta Part A 71 (2008) 779–785
Solvent effects on the infrared spectra of -alkoxyvinyl methyl ketones I. Carbonyl and vinyl stretching vibrations Sergey I. Vdovenko ∗ , Igor I. Gerus, Valery P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, Str. Murmanska 1, 02094 Kiev, Ukraine Received 21 November 2007; received in revised form 22 January 2008; accepted 30 January 2008
Abstract Infrared spectroscopy studies of six -alkoxyvinyl methyl ketones, with common structure R1 O–CR2 CH–COR3 , where R1 = R3 = CH3 , R2 = H (1); R1 = C2 H5 , R2 = H (2); R3 = CF3 ; R1 = R2 = CH3 , R3 = CF3 (3); R1 = C2 H5 , R2 = C6 H5 , R3 = CF3 (4); R1 = C2 H5 , R2 = 4-O2 NC6 H4 , R3 = CF3 (5); R1 = C2 H5 , R2 = C(CH3 )3 , R3 = CF3 (6) in 11 pure organic solvents of different polarity were undertaken to investigate the solute–solvent interactions and to correlate solvent properties by means of linear solvation energy relationships (LSER) with the carbonyl and vinyl stretching vibrations of existing stereoisomeric forms. It was shown that contrary to simple carbonyl-containing compounds where solvent HBD acidity (α) has the largest influence on the ν˜ (C O) band shift to lower wavenumbers, the dipolarity/polarizability (π*) term plays the main role in the interactions of conjugated enones with solvent molecules leading to the ν˜ (C O) and ν˜ (C C) bathochromic band shifts. The trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with solvents. For the ν˜ (C C) band of non-fluorinated enone 1 solvent HBD acidity (α) and solvent HBA basicity term (β) play a perceptible role, whereas for 2 these terms are not significant. -Substituents in fluorinated enones such as R2 = H, C6 H5 , and C(CH3 )3 assist in the intermolecular hydrogen bond formation of the carbonyl moiety with HBD solvents, while -substituents such as CH3 and 4-NO2 C6 H4 prevent the C O group to form the H-bonds with HBD solvents (the solvent HBD acidity term (α) is not significant). The comparison of four conformers of the enone 1 reveals that (EEE) form is the most polarizable conformer; the influences of the solvent dipolarity/polarizability (π*) and solvent HBD acidity (α) term on the bathochromic ν˜ (C O) band shift are opposite to one another. © 2008 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; -Alkoxyvinyl methyl ketones; Solvent influence; Linear solvation energy relationship
1. Introduction The vibrational spectrum of a molecule depends not only on the strength of a certain bond, but also can be markedly influenced by medium factors. The influence of the solvent on spectral properties of molecules, generally referred to as solvatochromism, have been investigated for many years, generating a copious literature [1]. Particularly in the case of vibrational spectra, it is known that their wavenumber values, intensities, and band shapes are influenced by the nature of the solvent via dielectric effects and specific interactions. In this context, molecules bearing the carbonyl moiety have been extensively investigated, as their dipolarity and hydrogen bond accepting nature makes the carbonyl stretching mode very sensitive to the solvent influence. The same is true for the ν˜ (C C) mode [2,3]. Several approaches
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have been used in attempts to characterize, correlate, and predict the solvent influences on spectral properties. One empirical approach uses correlative methods in the framework of quantitative structure–activity relationships (QSAR). An example is the linear solvation energy relationship (LSER) concept developed by Kamlet et al. [4], which, in turn, is based on linear free energy relationships (LFER). The LSER approach finds an equation, P = ci pi , relating some empirical property, P, to a set of parameters, {pi }, which have molecular structural interpretations; statistics (often multilinear regression) are used to find the coefficients, {ci }. In general terms, the LSER model can be written as Property = dipolarity/polarizability + hydrogen bond donor acidity + hydrogen bond acceptor basicity + bulk/cavity (1)
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Scheme 1. Molecular structures of the enones studied.
The model equation of LSER applied in infrared spectroscopy is shown in the following equation: ν˜ = ν˜ 0 + (sπ∗ + dδ) + aα + bβ + hδ2H
(2)
where ν˜ is the vibration wavenumber of a solute (such as ν˜ (C O) or ν˜ (C C)) in pure solvents and ν˜ 0 is the regression value of ν˜ (C O) or ν˜ (C C) in cyclohexane as reference solvent, π* is an index of the solvent’s dipolarity/polarizability, δ is a discontinuous polarizability correction term for polychlorinated aliphatic and aromatic solvents, α is a measure of the solvent’s hydrogen bond donor (HBD) acidity, β is a measure of the solvent’s hydrogen bond acceptor (HBA) basicity, and δH is the Hildebrand’s solubility parameter. The regression coefficients s, d, a, b, and h in Eq. (2) measure the relative susceptibilities of the solvent-dependent vibration wavenumber of the solute to the proper solvent parameters. The solvent influence on the ν˜ (C O) stretching mode was already investigated for various carbonyl-containing compounds to verify the influence of different solvent parameters [2,5–11], Table 1 Solvatochromic parameters and Hildebrand’s solubility parameter δH [11,20] for 11 solvents No.
Solvent
δ2H
π*
β
α
1 2 3 4 5 6 7 8 9 10 11
Cyclohexane n-Hexane Di-n-butyl ether Tetrahydrofuran 1,4-Dioxane Methanol 2-Propanol 1-Butanol 2,2,2-Trifluoroethanol Acetonitrile Dimethyl sulfoxide
0.67 0.55 0.97 0.83 1.00 2.10 1.32 1.30 1.371 1.42 1.44
0.00 –0.08 0.24 0.58 0.55 0.60 0.48 0.47 0.73 0.75 1.00
0.00 0.00 0.46 0.55 0.37 0.62 0.95 0.88 0 0.31 0.76
0.00 0.00 0.00 0.00 0.00 0.93 0.76 0.79 1.51 0.19 0.00
whereas for the ν˜ (C C) stretching mode of unsaturated ketones these investigations are not so numerous [2,3]. With hydroxylic solvents, such as alkyl alcohols, carbonyl compounds are known to form intermolecular hydrogen bonds [12]. Similarly, the C C bond, possessing a system, can be a guest in intermolecular hydrogen bonding too [13]. In this work, we studied the influence of the molecular structure of different -alkoxyvinyl methyl ketones on the solute–solvent interactions. The present study was performed to investigate the solvent-induced effects of 11 solvents of different polarity on the infrared spectra of -alkoxyvinyl methyl ketones and to correlate solvent properties with the band shifts of ν˜ (C O) and ν˜ (C C) stretching vibrations. Recently we discussed the conformational behavior of five trifluoroacetyl-␣,-enones 2–6 and their non-fluorinated analogue 1 [14]. We showed that all studied enones are able to generate various stereoisomeric forms due to rotational isomerism around the vinylic C C double bond, the C C single bond between vinyl and carbonyl group, and the C O single bond between vinyl and alkoxy group (see Scheme 1). We also evaluated the molar percentage of each form existing in equilibrium with each other. Moreover, the enones studied showed a remarkable dependence of their reactivity on the nature of the solvents under nucleophilic substitution reactions [15]. From this point of view it was interesting to investigate solvent effects on each conformer of these enones. 2. Experimental 2.1. General n-Hexane (obtained from Aldrich) was purified using standard techniques and was dried over the appropriate drying agent before use. Acetonitrile (Merk, Uvasol) was purified additionally by a four-step method as previously described [15,16],
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stored under N2 , and distilled prior to use. All enones were stored under dry nitrogen at +4 ◦ C and were purified by distillation before use.
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and ν˜ (C C) of stereoisomeric enone forms were estimated after band separation similar to that carried out by us previously [14]. 3. Results and discussion
2.2. Synthesis of the studied enones 1–6 Compounds 2–5 were prepared as described in works [17–19]. Enone 1 was commercial product (Fluka). Enone (3Z)4-ethoxy-1,1,1-trifluoro-5,5-dimethylhex-3-en-2-one (6) was obtained by applying the procedure described for 2 [18,19] using 2-ethoxy-3,3-dimethylbut-1-ene and trifluoroacetic acid anhydride [14]. 2.3. Infrared spectra Infrared spectra were recorded on infrared spectrophotometer Specord M80 (Carl Zeiss Jena) at the room temperature (20 ± 2 ◦ C). Solution concentrations were 0.11 M, cell width 0.013 cm. Infrared spectra of enones 2–6 were registered in region 1800–1500 cm−1 , whereas IR spectra of 1 were registered in the region 1750–1550 cm−1 . The wavenumbers ν˜ (C O)
The Kamlet–Taft solvatochromic parameters π*, α, β, and Hildebrand’s solubility parameter δH for 11 solvents used are listed in Table 1. The wavenumbers ν˜ (C O) and ν˜ (C C) of the stereoisomeric forms of the enones 1–6 measured in 11 solvents are summarized in Table 2. It should be noted that the ν˜ (C O) and ν˜ (C C) vibrations in ␣,-unsaturated enones are mixed [2,21], but as previously described [14], for simplicity we denote the vibration for which the largest contribution stems from the carbonyl mode as ν˜ (C O), whereas ν˜ (C C) stands for the vibration, in which the contribution of the vinyl mode prevails. We correlated the wavenumbers with the solvatochromic parameters of 11 solvents (Table 1) according to Eq. (2) and present the results in Table 3 as regression coefficients of parameters π*, α, β, and δ2H for stereoisomeric forms of enones 1–6. Only terms at the 0.95 significance level or higher were retained. The quality of multiple linear regressions is indicated by the standard error
Table 2 The ν˜ (C O) and ν˜ (C C) values of enones 1–6 in 11 solvents Enone
Stereoisomeric form
EZE EZZ 1
EEE EEZ EZZ EEE
2
EEZ EZZ 3
EZE ZZZ EZZ
4
EZE EZZ
5
ZZZ 6
EZZ a b
Vibration, ν˜ (cm−1 )
Solventa 1
2
3
4
5
6
7
8
9
10
11
C C C C C C C C
O C O C O C O C
1698 1617 1684 1602 1675 1647 1662 1626
1700 1618 1686 1603 1676 1649 1662 1626
1699 1616 1684 1602 1674 1647 1661 1625
1692 1612 1682 1599 1669 1623 1658 1624
1690 1611 1683 1598 1670 1638 1657 1624
1695 1601 1681 1591 1672 1639 1649 1623
1698 1603 1683 1592 1669 1623 1649 1623
1695 1602 1682 1591 1672 1622 1649 1622
1679 1603 1667 1588 1661 1623 1650 1623
1689 1610 1675 1598 1668 1639 1655 1625
1686 1606 1678 1594 1664 1636 1654 1621
C C C C C C
O C O C O C
1722 1598 1704 1639 1699 1617
1723 1599 1706 1640 1701 1618
1723 1597 1704 1638 1699 1615
1716 1593 1705 1635 1689 1610
1718 1594 1696 1635 1690 1610
1716 1597 1700 1637 1693 1614
1716 1594 1703 1634 1695 1614
1717 1594 1703 1635 1695 1613
1713 1588 1691 1634 1676 1614
1714 1592 1689 1634 1682 1609
1708 1588 1684 1629 1675 1614
C C C C
O C O C
1716 1588 1707 1573
1717 1589 1708 1573
1713 1585 1701 1574
1708 1582 1697 1574
1709 1582 1700 1574
1709 1582 1699 1571
1711 1583 1699 1571
1711 1584 1699 1575
1709 1572 1700 1568
1707 1580 1698 1572
1703 1573 1695 1568
C C C C C C
O C O C O C
1722 1588 1714 1566 1697 1556
1722 1590 1715 1567 1697 1557
1722 1588 1713 1564 1694 1557
1717 1587 1709 1564 1690 1555
1720 1586 1708 1563 1688 1557
1714 1587 1707 1564 1681 1555
1716 1587 1709 1564 1682 1556
1714 1587 1711 1563 1679 1554
1709 1586 1699 1563 1671 1554
1714 1586 1704 1563 1687 1554
1716 1584 1701 1563 1682 1555
C O C C
insb
insb
1716 1574
1710 1571
1711 1572
1711 1571
1713 1571
1712 1572
1702 1565
1708 1570
1703 1567
C C C C
1708 1579 1703 1565
1709 1581 1704 1566
1706 1576 1701 1560
1702 1572 1694 1557
1703 1571 1696 1557
1703 1577 1695 1559
1705 1573 1695 1558
1705 1572 1695 1558
1691 1566 1679 1553
1699 1571 1692 1555
1696 1567 1690 1552
O C O C
Solvent numeration is the same as in Table 1. Insoluble in this solvent.
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Table 3 The coefficients (s, a, b, and h) of the correlation equations ν˜ (C − X) = ν˜ (C X)0 + sπ* + aα + bβ + hδ2H , errors of the estimate (S.D.), Fisher indexes of reliability (F), and correlation coefficients (R) Enone
Conformer
ν˜ (C X)
ν˜ (C X)0 (cm−1 )
s
a
b
h
N
S.D.
F
R
EZE
C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
1694 1620 1682 1603 1672 1642 1663 1626 1722 1596 1708 1638 1698 1611 1716 1591 1707 NAb 1719 1589 1695 1566 1715 1555 1720 1578 1709 1575 1701 1564
−23.139 −4.459 −14.057 −5.454 −14.876 −11.427 −7.378 −2.812 −15.750 −12.195 −19.552 −9.396 −31.217 −15.741 −12.600 −9.008 −8.722 NA −12.242 −4.245 −12.843 −3.024 −13.883 −5.291 −16.944 −8.102 −12.421 −15.197 −18.680 −11.753
−3.964 −6.964 −5.600 −7.604 −4.957 −16.002 −7.149 −1.049 −2.303 NS NS NS NS −7.368 NS NS NS NA −9.991 NS −10.266 NS NS −4.003 NS NS NS NS −5.795 NS
8.728 −6.535 8.551 −3.410 NS −15.409 −4.341 −2.394 NSa NS NS NS NS NS NS NS −5.334 NA NS NS −3.960 NS NS NS −1.024 −2.453 NS −2.530 NS −2.935
6.075 −2.430 2.302 NS 5.597 7.958 NS NS 2.536 4.237 NS 2.412 3.560 7.481 NS −3.316 NS NA 3.821 NS 2.300 NS −1.030 2.406 0.357 0.48 NS 5.764 4.906 NS
11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 – 11 11 11 11 11 11 9 9 11 11 11 11
1.3005 1.5135 1.7006 1.2815 0.9413 6.3552 1.6161 0.8499 2.0045 0.9915 4.2908 1.1979 2.2911 2.3733 0.3769 1.1196 1.1489 – 1.1158 0.7053 1.6926 0.8781 0.9346 0.9747 0.5545 0.3645 0.6511 1.2443 0.4014 0.7827
56.553 42.372 22.905 39.853 51.768 6.2335 31.223 7.3724 10.628 23.267 5.2464 14.151 27.308 2.3185 277.45 34.173 27.936 – 20.601 9.8055 33.833 4.2284 47.599 2.0126 82.348 51.953 81.541 25.052 300.47 64.086
0.987 0.943 0.969 0.982 0.986 0.898 0.977 0.912 0.946 0.974 0.900 0.959 0.978 0.806 0.998 0.982 0.978 – 0.971 0.942 0.982 0.879 0.987 0.785 0.995 0.967 0.992 0.976 0.998 0.990
EZZ 1
EEE EEZ EZZ EEE
2
EEZ EZZ 3
EZE ZZZ EZZ
4
EZE EZZ
5
ZZZ 6
EZZ a b
O C O C O C O C O C O C O C O C O C O C O C O C O C O C O C
Eq. no. (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31)
Not significant. Not applicable.
of the estimate (S.D., the smaller the better), the Fisher index of reliability (F, the larger the better), and the correlation coefficient (R, variance = R2 , the closer to 1 the better). A correlation equation was considered as acceptable if the product correlation coefficient, R, indicates that the equation accounts for more than 80% of the variance (R2 ≥ 0.80). It is reasonable to consider the multiple correlations for enones 1–6 separately and then to compare the results obtained. Examination of Table 3 reveals some general features: for all stereoisomeric forms of studied enones statistically significant equations (R2 ≥ 0.80) are observed, the ν˜ (C C) band of the (EZE) stereoisomer of 3 being the exception as result of considerable overlapping of the ν˜ (C C) bands of the two stereoisomers [viz. (EZE) and (EZZ)] of the enone 3. Another similarity is that not all parameters of Eq. (2) are statistically significant. The enone wavenumbers are mainly related to the solvent’s dipolarity/polarizability parameter π* and to a much lesser extent to the solvent’s HBD acidity α, and to the solvent’s HBA basicity β.
of all four conformers exerts the dipolarity/polarizability term (π*): its regression coefficient s diminishes in absolute value almost synchronously with ν˜ (C O)0 (Fig. 1, line 䊉), showing that the lower the electron population of the carbonyl bond the smaller influence of the solvent’s dipolarity/polarizability (π*) is. Moreover, the coefficient s is negative, indicating ν˜ (C O) shift to lower wavenumbers with an increase of the solvent dipo-
3.1. Enone 1 Enone 1 equilibrates to four E-conformers (viz. EZE, EZZ, EEE and EEZ) with a percentage 43, 33, 10 and 14 mol.%, respectively. The largest influence on the ν˜ (C O) vibrations
Fig. 1. Plot of the coefficients s (䊉) and a () vs. ν˜ (C O)0 for MBO conformers (see Tables 2 and 3).
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larity/polarizability. Besides the dielectric effects (measured by π*), solvent molecules interact directly with the C O (and C C) group by two kinds of attractive forces. The first of these forces is that the solvent molecule forms a hydrogen bond with oxygen of the C O (or with system of this double bond). The capability of solvent to donate proton in a solvent-to-solute (enone) hydrogen bond is described by parameter α, solvent HBD acidity. The second is that the solvent molecule which has electronegative atoms O, N and S can form hydrogen bonds with a solute as proton donor. Parameter β describes the ability of a solvent to accept a proton in a solute-to-solvent hydrogen bond, i.e. its hydrogen bond acceptor capability with respect to a reference hydrogen bond donor. In enone 1 the formation of H-bond with vinylic ␣-hydrogen atom is possible owing to nucleophilic attack of solvent’s electronegative atoms. Solvent HBD acidity (α) has the perceptible effect on the ν˜ (C O) mode of (EZE), (EZZ) and (EEE) conformers. Increasing HBD acidity is associated with decreasing wavenumber. This is consistent with decreasing C O bond strength as modeled by the force constant, k, √ in the model for harmonic oscillator wavenumber, ν˜ = (2c)−1 kμ. Moreover, as can be seen from Fig. 1 (line ) the more polarizable carbonyl moiety is [the less wavenumber of ν˜ (C O) mode] the more significant is the solvent HBD acidity (α). From this point of view the (EEZ) conformer is the most suitable for hydrogen bond formation between the carbonyl group of the solute and a solvent molecule. This suggests these equations to be physically reasonable. From physical standpoint it should be expected that the coefficient b of the solvent HBA basicity (β) should be negative too, but in Eqs. (3) and (5) it is highly positive. The increase in wavenumber with increasing solvent HBA basicity suggests the physical reasonableness since the increase of oscillating mass shifts the ν˜ (C O) band to lower wavenumbers, therefore these positive values of b are apparent and are evidence for a lack of that kind of interaction with (EZE), (EZZ) and (EEE) conformers. In (EEZ) conformer the b value is negative and rather significant. The only possibility of the ν˜ (C O) shift to high wavenumbers with an increase of solvent HBA basicity (β) is the rotation of the carbonyl group out of the C C double bond plane (with disturbance of the conjugation between carbonyl and vinyl double bonds) as a result of the carbonyl specific solvation with solvent molecules, but at present we have no evidence to confirm this assumption. Contrary to the ν˜ (C O) the main influence on vibrations of the vinyl moiety makes the solvent HBD acidity (α) [a/s = 1.56 (EZE), Eq. (8); 1.39 (EZZ), Eq. (4); 1.40 (EZZ), Eq. (6)] with the exception of (EEZ) [a/b = 0.37, Eq. (8)] where the coefficient a is the smallest. Hence, the (EEZ) conformation is not convenient for a H-bond formation between the C C double bond and HBD solvents and the hydrogen bonding with these solvents takes place in the (EEZ) conformer presumably on the carbonyl moiety. All a values are negative, pointing out the ν˜ (C C) to decrease with increasing solvent HBD acidity (α). The same is true for s and b: they are negative too, so an increase of all three parameters, π*, α, and β, shifts the ν˜ (C C) to lower wavenumbers. A comparison of Eqs. (3)–(10) shows that the ν˜ (C O) band for all four conformers of MBO are shifted to lower wavenum-
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bers primarily due to dipole–dipole interactions with solvent molecules, while the ν˜ (C C) band shift to lower wavenumbers occurs to a greater extent due to the solvent HBD acidity (α). The vinylic vibrations are more sensitive to solvent HBD acidity (α) than that of ν˜ (C O) [with exception of the (EEZ) conformer]. 3.2. Enone 2 The replacement of the methyl group in 1 by CF3 in 2 increases the difference ˜ν between the ν˜ (C O) and the ν˜ (C C) band due to the high electron-withdrawing ability of the trifluoromethyl group. On the one hand, it increases the electron density at the C O double bond and hence shifts the ν˜ (C O) band to higher wavenumbers. On the other hand, on account of the higher electron withdrawal from the C C moiety by the trifluoromethyl carbonyl group, this produces a bathochromic shift of the ν˜ (C C) [14]. Simultaneously the ν˜ (C O) intensities A go down with an increase of the ν˜ (C C) intensities (in 1 the intensities A of the ν˜ (C O) and ν˜ (C C) are almost equal, whereas in 2 the A(C O) is 8–10-fold higher then the A(C C) [14,21]). Hence, the bond order of the C O double bond increases and that of the C C bond decreases as result of a weakening of the conjugation in the C C C O system [22]. For 2 there are three existing conformers of E-isomer (EZZ), (EEE) and (EEZ), with percentage 71, 23 and 6 mol.%, respectively [14]. The largest influence on ν˜ (C O) of all conformers has the solvent’s dipolarity/polarizability (π*): the higher the ν˜ (C O)0 of the conformers the higher is in absolute value the coefficient s (see Table 3). Solely for (EZZ), the coefficient a is retained [it is small and negative, Eq. (11)], while for other conformers it is not significant on the 0.95 significance level. The solvent HBA basicity term is insignificant for all three conformers. Similarly to the ν˜ (C O) absorption, the solvent dipolarity/polarizability (π*) term has the largest influence on the ν˜ (C C) absorption, whereas the solvent HBD acidity and HBA basicity terms are insignificant in Eqs. (12) and (14). Exception is Eq. (16) in which coefficient a is highly negative indicating that the hydrogen bond formation has appreciable effect on the ν˜ (C C) band of (EEZ). 3.3. Enone 3 Steric hindrance by the methyl group in -position of the 3 vinyl moiety reduces the number of conformers of E-isomer to two, namely (EZZ) and (EZE), the former being much more populated (92 mol.%). As can be easily seen from Table 3, solvent dipolarity/polarizability (π*) influences mainly on the stretching mode ν˜ (C O) of both conformers, while the solvent HBD acidity and HBA basicity terms are insignificant [Eqs. (20) and (22)] with exception of the (EZE) conformer, for which the solvent HBA basicity term is significant and highly negative. For the ν˜ (C C) band Eq. (1) is only applicable for the (EZZ) conformer. According to Eq. (18) in Table 3, the solvent dipolarity/polarizability (π*) term is highly negative, while other terms are insignificant. Hence, for the (EZZ) and (EZE) conformers of 3, like the ones of 2, the main contribution according to Eq. (1) makes the solvent dipolarity/polarizability (π*) term.
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3.4. Enone 4 When -CH3 is replaced by -C6 H5 the Z isomer [namely, the (ZZZ) conformer] appears in the enone 4 with appreciable percentage (35 mol.% in n-hexane) [14]. It is interesting to note that the coefficient s in Eqs. (20), (22), and (24) for the ν˜ (C O) band is highly negative and close to each other. Moreover, for the (ZZZ) and (EZZ) stereoisomeric forms the coefficient a is also highly negative, indicating the C O ability to form H-bonds with HBD solvents, while for (EZE) (Eq. (24)) this coefficient is insignificant. Hence, the bathochromic ν˜ (C O) band shift in the (ZZZ) and (EZZ) stereoisomers is mainly due to dipole–dipole interactions and H-bond formation with solvent molecules. The contribution of solvent basicity term (β) is insignificant in Eqs. (20)–(25), except in Eq. (22) where coefficient b is small in comparison with coefficients s and a. Hence, in both forms carbonyl and vinyl stretching vibrations are not susceptible to the influence of HBA solvents. 3.5. Enone 5 The strong electron-withdrawing nitro group in the 4-position of the benzene ring in 5 reduces a number of stereoisomers to only one, viz. (EZZ). A principal contribution to Eqs. (26) and (27) makes the solvent dipolarity/polarizability (π*) term, while neither the ν˜ (C O) nor the ν˜ (C C) vibrations are susceptible to solvent hydrogen bond acceptor basicity, differing from the corresponding conformer of 4 where the ν˜ (C O) vibration is highly sensitive to this term. Hence, the 4-NO2 C6 H4 -substituent, in contrast to C6 H5 -, decreases the negative charge at the oxygen atom of the carbonyl moiety, so it becomes unable to form hydrogen bonds with HBD solvents. 3.6. Enone 6 The distinguishing feature of this enone is the formation of two stereoisomeric forms (ZZZ) and (EZZ), the former being more populated (74 and 26% in n-hexane, respectively [14]). As in the enones described previously, the basic influence on the ν˜ (C O) and ν˜ (C C) band has the solvent polarity/polarizability (π*), whereas the solvent HBD acidity term (α) is insignificant [moderately negative for ν˜ (C O) of (EZZ) only, see (Eq. (30)]. From a comparison of Eqs. (20) and (21) and Eqs. (28) and (29) for the (ZZZ) stereoisomer of 3 and 5 one can conclude that the t-butyl group in -position of 6 prevents the carbonyl moiety to form H-bonds with solvent molecules, while the C O group of (EZZ) conformer forms hydrogen bonds with HBD solvents [a = −5.8, Eq. (30)]. The C C bond of the (EZZ) stereoisomer is shielded by the -substituent in both enones, 4 and 6. In contrast to the (ZZZ) stereoisomer of 6, the ν˜ (C O) band of (EZZ) form of 6, similar to the corresponding stereoisomeric form of 4, is shifted to the lower wavenumbers due to H-bond formation [cf. a in Eqs. (22) and (30)]. The coefficient b [Eqs. (29) and (31)] is significant for the ν˜ (C C) shift for both stereoisomers, indicating that the t-butyl substituent does not prevent from nucleophilic attack of basic solvent molecules on the positively charged -carbon of the C C double bond (or does not hinder in
H-bond formation between the solvent and vinylic ␣-hydrogen atom). Accordingly, a common feature of Eqs. (3)–(31) is the fact that dipolarity/polarizability plays the main role in the interactions of enones with solvent molecules, which leads to the observed bathochromic shifts of the ν˜ (C O) and ν˜ (C C) bands. The difference between the ν˜ (C O) band of solutes containing unconjugated carbonyl group and enones, where the C O double bond is conjugated with the vinyl moiety, is that the dominating solvent influence on former has the solvent HBD acidity [11], while for the enones solvent dipolarity/polarizability prevails. The insignificance of the solvent HBA basicity term for fluorinated enones is another general property of most equations listed in Table 3. In all enones (excepting 6) the conformer (EZZ) is most populated. Therefore, it is reasonable to compare the results obtained for this conformer of the enones examined. The trifluoromethyl group lowers the C O polarity, so the trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with HBD solvents (a/s = 0.40, 1 and 0.15, 2). Moreover, for ν˜ (C C) of the enone 1 the solvent HBD acidity (α) and the solvent HBA basicity term (β) play a perceptible role [a = −7.604, b = −3.410, Eq. (6)], while for 2 these terms are not significant (Eq. (12)). Hence, the system of the C C double bond in 2 is so polarized that it is unable to form hydrogen bonds with HBD solvents and the vinylic ␣-hydrogen atom does not participate in H-bonding with HBA solvent molecules. Based on the influence of -substituents on the ν˜ (C C) band shift one can divide -substituents into two kinds: -H, -C6 H5 , and -C(CH3 )3 assist in hydrogen bond formation with HBD solvents [coefficient a is negative and large enough, Eqs. (11), (22) and (30)], whereas -CH3 and -(4-NO2 C6 H4 ) prevent the C O group to form H-bonds with such solvents [the solvent HBD acidity term is not significant, Eqs. (17) and (26)]. The substituent -C6 H5 is a -donor which increases the negative charge on the carbon oxygen, thus promoting H-bond formation; -C(CH3 )3 interacts sterically with the carbonyl moiety of the trifluoroacetyl group, which rotates out of the C C double bond plane with weakening the C C C O conjugation and thus increasing the negative charge on carbonyl oxygen atom, as consequence. In the enone 1 all four existing conformers of the E-isomer are realized so it is desirable to compare the data for these conformers according to which EEE is the most polarizable, having high negative s, a, and b coefficients not only for the ν (C O) multiple linear regression but also for the same correlation of ν˜ (C C). It follows from Fig. 1 that the slopes of coefficients s and a are opposed to each other, hence the larger influence of the solvent dipolarity/polarizability has on ν˜ (C O) a smaller effect of the solvent HBD acidity on ν˜ (C O) is. There is no similar dependence for ν (C C), probably due to the existence of more than one center of H-bond formation. 4. Conclusion Contrary to simple carbonyl-containing molecules where the solvent HBD acidity has the largest influence on the ν˜ (C O) band shift to lower wavenumbers, the dipolarity/polarizability
S.I. Vdovenko et al. / Spectrochimica Acta Part A 71 (2008) 779–785
(π*) term plays the main role in the interactions of conjugated enones with solvent molecules and leads to bathochromic shift of the ν (C O) and ν (C C) bands. For all four conformers of 1 the ν˜ (C O) bands are shifted to the lower wavenumbers primarily due to dipole–dipole solute–solvent interactions, while the bathochromic ν˜ (C C) band shift occurs to a greater extent due to the solvent HBD acidity (α). The trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with HBD solvents in comparison to the acetyl group. For the ν˜ (C C) band of the non-fluorinated enone 1 the solvent HBD acidity (α) and solvent HBA basicity term (β) play a perceptible role whereas for 2 these terms are not significant. -Substituents in fluorinated enones such as H, C6 H5 , and C(CH3 )3 assist in the hydrogen bond formation with HBD solvents, while -CH3 and 4-NO2 C6 H4 prevent the C O to form H-bonds with these solvents (the solvent HBD acidity term is not significant). The comparison of four conformers of the enone 1 reveals that (EEE) is most polarizable conformer. The influence of the solvent dipolarity/polarizability (π*) term and solvent HBD acidity (α) term on the bathochromic ν˜ (C O) band shift are opposite to each other. References [1] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 3rd ed., Wiley-VCH, 2003. [2] L.C.J. Almeida, P.S. Santos, Spectrochim. Acta: A 58 (2002) 3139.
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[3] J. Zhen, Q. Liu, D. Fang, Spectrochim. Acta: A 60 (2004) 3119. [4] M.J. Kamlet, R.M. Doherty, M.H. Abraham, R.A. Taft, Quant. Struct.–Act. Relat. 7 (1988) 71. [5] R.A. Nyquist, H.A. Fouchea, G.A. Hoffman, D.L. Hasha, Appl. Spectrosc. 45 (1991) 860. [6] R.A. Nyquist, C.L. Putzig, D.L. Hasha, Appl. Spectrosc. 43 (1989) 23. [7] R.A. Nyquist, Appl. Spectrosc. 43 (1989) 1049. [8] R.A. Nyquist, T.D. Clark, R. Streck, Vib. Spectrosc. 7 (1994) 275. [9] P. Bruni, E. Giorginy, E. Maurelli, G. Tosi, Vib. Spectrosc. 12 (1996) 249. [10] R.A. Nyquist, C.L. Putzig, T.D. Clark, Vib. Spectrosc. 12 (1996) 81. [11] J.B.F.N. Engberts, G.R. Famini, A. Perjessy, L.Y. Wilson, J. Phys. Org. Chem. 11 (1998) 261. [12] F. Besseau, M. Luc¸on, C. Laurence, M. Berthelot, J. Chem. Soc., Perkin Trans. 2 (1998) 101. [13] F. Besseau, C. Laurence, M. Berthelot, Bull. Soc. Chim. Fr. 133 (1996) 381. [14] S.I. Vdovenko, I.I. Gerus, N.V. Lutenko, V.P. Kukhar, J. W´ojcik, J. Mol. Struct. 840 (2007) 125. [15] S.I. Vdovenko, I.I. Gerus, J. W´ojcik, J. Phys. Org. Chem. 14 (2001) 533. [16] J.A. Riddick, W.B. Bunger, T.K. Sakano, Organic Solvents—Physical Properties and Methods, 4th ed., Wiley-Interscience, New York, 1986. [17] I.I. Gerus, L.M. Kacharova, S.I. Vdovenko, Synthesis (2001) 431. [18] M. Hojo, R. Masuda, E. Okada, Synthesis (1986) 1013. [19] M.A.P. Martins, G.M. Siquera, G.P. Bastos, H.G. Bonacorso, N. Zanatta, J. Heterocycl. Chem. 33 (1996) 1619. [20] M.H. Abraham, R.M. Doherty, M.J. Kamlet, J.M. Harris, R.W. Taft, J. Chem. Soc., Perkin Trans. 2 (1978) 913. [21] J. D˛abrowski, M. Tencer, Bull. Chem. Soc. Jpn. 48 (1975) 1310. [22] S.I. Vdovenko, I.I. Gerus, M.G. Gorbunova, J. Chem. Soc., Perkin Trans. 2 (1993) 559.
Spectrochimica Acta Part A 71 (2008) 779–785
Solvent effects on the infrared spectra of -alkoxyvinyl methyl ketones I. Carbonyl and vinyl stretching vibrations Sergey I. Vdovenko ∗ , Igor I. Gerus, Valery P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, Str. Murmanska 1, 02094 Kiev, Ukraine Received 21 November 2007; received in revised form 22 January 2008; accepted 30 January 2008
Abstract Infrared spectroscopy studies of six -alkoxyvinyl methyl ketones, with common structure R1 O–CR2 CH–COR3 , where R1 = R3 = CH3 , R2 = H (1); R1 = C2 H5 , R2 = H (2); R3 = CF3 ; R1 = R2 = CH3 , R3 = CF3 (3); R1 = C2 H5 , R2 = C6 H5 , R3 = CF3 (4); R1 = C2 H5 , R2 = 4-O2 NC6 H4 , R3 = CF3 (5); R1 = C2 H5 , R2 = C(CH3 )3 , R3 = CF3 (6) in 11 pure organic solvents of different polarity were undertaken to investigate the solute–solvent interactions and to correlate solvent properties by means of linear solvation energy relationships (LSER) with the carbonyl and vinyl stretching vibrations of existing stereoisomeric forms. It was shown that contrary to simple carbonyl-containing compounds where solvent HBD acidity (α) has the largest influence on the ν˜ (C O) band shift to lower wavenumbers, the dipolarity/polarizability (π*) term plays the main role in the interactions of conjugated enones with solvent molecules leading to the ν˜ (C O) and ν˜ (C C) bathochromic band shifts. The trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with solvents. For the ν˜ (C C) band of non-fluorinated enone 1 solvent HBD acidity (α) and solvent HBA basicity term (β) play a perceptible role, whereas for 2 these terms are not significant. -Substituents in fluorinated enones such as R2 = H, C6 H5 , and C(CH3 )3 assist in the intermolecular hydrogen bond formation of the carbonyl moiety with HBD solvents, while -substituents such as CH3 and 4-NO2 C6 H4 prevent the C O group to form the H-bonds with HBD solvents (the solvent HBD acidity term (α) is not significant). The comparison of four conformers of the enone 1 reveals that (EEE) form is the most polarizable conformer; the influences of the solvent dipolarity/polarizability (π*) and solvent HBD acidity (α) term on the bathochromic ν˜ (C O) band shift are opposite to one another. © 2008 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; -Alkoxyvinyl methyl ketones; Solvent influence; Linear solvation energy relationship
1. Introduction The vibrational spectrum of a molecule depends not only on the strength of a certain bond, but also can be markedly influenced by medium factors. The influence of the solvent on spectral properties of molecules, generally referred to as solvatochromism, have been investigated for many years, generating a copious literature [1]. Particularly in the case of vibrational spectra, it is known that their wavenumber values, intensities, and band shapes are influenced by the nature of the solvent via dielectric effects and specific interactions. In this context, molecules bearing the carbonyl moiety have been extensively investigated, as their dipolarity and hydrogen bond accepting nature makes the carbonyl stretching mode very sensitive to the solvent influence. The same is true for the ν˜ (C C) mode [2,3]. Several approaches
∗
Corresponding author. Tel.: +380 44 573 25 93; fax: +380 44 573 25 52. E-mail address: [email protected] (S.I. Vdovenko).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.01.031
have been used in attempts to characterize, correlate, and predict the solvent influences on spectral properties. One empirical approach uses correlative methods in the framework of quantitative structure–activity relationships (QSAR). An example is the linear solvation energy relationship (LSER) concept developed by Kamlet et al. [4], which, in turn, is based on linear free energy relationships (LFER). The LSER approach finds an equation, P = ci pi , relating some empirical property, P, to a set of parameters, {pi }, which have molecular structural interpretations; statistics (often multilinear regression) are used to find the coefficients, {ci }. In general terms, the LSER model can be written as Property = dipolarity/polarizability + hydrogen bond donor acidity + hydrogen bond acceptor basicity + bulk/cavity (1)
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Scheme 1. Molecular structures of the enones studied.
The model equation of LSER applied in infrared spectroscopy is shown in the following equation: ν˜ = ν˜ 0 + (sπ∗ + dδ) + aα + bβ + hδ2H
(2)
where ν˜ is the vibration wavenumber of a solute (such as ν˜ (C O) or ν˜ (C C)) in pure solvents and ν˜ 0 is the regression value of ν˜ (C O) or ν˜ (C C) in cyclohexane as reference solvent, π* is an index of the solvent’s dipolarity/polarizability, δ is a discontinuous polarizability correction term for polychlorinated aliphatic and aromatic solvents, α is a measure of the solvent’s hydrogen bond donor (HBD) acidity, β is a measure of the solvent’s hydrogen bond acceptor (HBA) basicity, and δH is the Hildebrand’s solubility parameter. The regression coefficients s, d, a, b, and h in Eq. (2) measure the relative susceptibilities of the solvent-dependent vibration wavenumber of the solute to the proper solvent parameters. The solvent influence on the ν˜ (C O) stretching mode was already investigated for various carbonyl-containing compounds to verify the influence of different solvent parameters [2,5–11], Table 1 Solvatochromic parameters and Hildebrand’s solubility parameter δH [11,20] for 11 solvents No.
Solvent
δ2H
π*
β
α
1 2 3 4 5 6 7 8 9 10 11
Cyclohexane n-Hexane Di-n-butyl ether Tetrahydrofuran 1,4-Dioxane Methanol 2-Propanol 1-Butanol 2,2,2-Trifluoroethanol Acetonitrile Dimethyl sulfoxide
0.67 0.55 0.97 0.83 1.00 2.10 1.32 1.30 1.371 1.42 1.44
0.00 –0.08 0.24 0.58 0.55 0.60 0.48 0.47 0.73 0.75 1.00
0.00 0.00 0.46 0.55 0.37 0.62 0.95 0.88 0 0.31 0.76
0.00 0.00 0.00 0.00 0.00 0.93 0.76 0.79 1.51 0.19 0.00
whereas for the ν˜ (C C) stretching mode of unsaturated ketones these investigations are not so numerous [2,3]. With hydroxylic solvents, such as alkyl alcohols, carbonyl compounds are known to form intermolecular hydrogen bonds [12]. Similarly, the C C bond, possessing a system, can be a guest in intermolecular hydrogen bonding too [13]. In this work, we studied the influence of the molecular structure of different -alkoxyvinyl methyl ketones on the solute–solvent interactions. The present study was performed to investigate the solvent-induced effects of 11 solvents of different polarity on the infrared spectra of -alkoxyvinyl methyl ketones and to correlate solvent properties with the band shifts of ν˜ (C O) and ν˜ (C C) stretching vibrations. Recently we discussed the conformational behavior of five trifluoroacetyl-␣,-enones 2–6 and their non-fluorinated analogue 1 [14]. We showed that all studied enones are able to generate various stereoisomeric forms due to rotational isomerism around the vinylic C C double bond, the C C single bond between vinyl and carbonyl group, and the C O single bond between vinyl and alkoxy group (see Scheme 1). We also evaluated the molar percentage of each form existing in equilibrium with each other. Moreover, the enones studied showed a remarkable dependence of their reactivity on the nature of the solvents under nucleophilic substitution reactions [15]. From this point of view it was interesting to investigate solvent effects on each conformer of these enones. 2. Experimental 2.1. General n-Hexane (obtained from Aldrich) was purified using standard techniques and was dried over the appropriate drying agent before use. Acetonitrile (Merk, Uvasol) was purified additionally by a four-step method as previously described [15,16],
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stored under N2 , and distilled prior to use. All enones were stored under dry nitrogen at +4 ◦ C and were purified by distillation before use.
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and ν˜ (C C) of stereoisomeric enone forms were estimated after band separation similar to that carried out by us previously [14]. 3. Results and discussion
2.2. Synthesis of the studied enones 1–6 Compounds 2–5 were prepared as described in works [17–19]. Enone 1 was commercial product (Fluka). Enone (3Z)4-ethoxy-1,1,1-trifluoro-5,5-dimethylhex-3-en-2-one (6) was obtained by applying the procedure described for 2 [18,19] using 2-ethoxy-3,3-dimethylbut-1-ene and trifluoroacetic acid anhydride [14]. 2.3. Infrared spectra Infrared spectra were recorded on infrared spectrophotometer Specord M80 (Carl Zeiss Jena) at the room temperature (20 ± 2 ◦ C). Solution concentrations were 0.11 M, cell width 0.013 cm. Infrared spectra of enones 2–6 were registered in region 1800–1500 cm−1 , whereas IR spectra of 1 were registered in the region 1750–1550 cm−1 . The wavenumbers ν˜ (C O)
The Kamlet–Taft solvatochromic parameters π*, α, β, and Hildebrand’s solubility parameter δH for 11 solvents used are listed in Table 1. The wavenumbers ν˜ (C O) and ν˜ (C C) of the stereoisomeric forms of the enones 1–6 measured in 11 solvents are summarized in Table 2. It should be noted that the ν˜ (C O) and ν˜ (C C) vibrations in ␣,-unsaturated enones are mixed [2,21], but as previously described [14], for simplicity we denote the vibration for which the largest contribution stems from the carbonyl mode as ν˜ (C O), whereas ν˜ (C C) stands for the vibration, in which the contribution of the vinyl mode prevails. We correlated the wavenumbers with the solvatochromic parameters of 11 solvents (Table 1) according to Eq. (2) and present the results in Table 3 as regression coefficients of parameters π*, α, β, and δ2H for stereoisomeric forms of enones 1–6. Only terms at the 0.95 significance level or higher were retained. The quality of multiple linear regressions is indicated by the standard error
Table 2 The ν˜ (C O) and ν˜ (C C) values of enones 1–6 in 11 solvents Enone
Stereoisomeric form
EZE EZZ 1
EEE EEZ EZZ EEE
2
EEZ EZZ 3
EZE ZZZ EZZ
4
EZE EZZ
5
ZZZ 6
EZZ a b
Vibration, ν˜ (cm−1 )
Solventa 1
2
3
4
5
6
7
8
9
10
11
C C C C C C C C
O C O C O C O C
1698 1617 1684 1602 1675 1647 1662 1626
1700 1618 1686 1603 1676 1649 1662 1626
1699 1616 1684 1602 1674 1647 1661 1625
1692 1612 1682 1599 1669 1623 1658 1624
1690 1611 1683 1598 1670 1638 1657 1624
1695 1601 1681 1591 1672 1639 1649 1623
1698 1603 1683 1592 1669 1623 1649 1623
1695 1602 1682 1591 1672 1622 1649 1622
1679 1603 1667 1588 1661 1623 1650 1623
1689 1610 1675 1598 1668 1639 1655 1625
1686 1606 1678 1594 1664 1636 1654 1621
C C C C C C
O C O C O C
1722 1598 1704 1639 1699 1617
1723 1599 1706 1640 1701 1618
1723 1597 1704 1638 1699 1615
1716 1593 1705 1635 1689 1610
1718 1594 1696 1635 1690 1610
1716 1597 1700 1637 1693 1614
1716 1594 1703 1634 1695 1614
1717 1594 1703 1635 1695 1613
1713 1588 1691 1634 1676 1614
1714 1592 1689 1634 1682 1609
1708 1588 1684 1629 1675 1614
C C C C
O C O C
1716 1588 1707 1573
1717 1589 1708 1573
1713 1585 1701 1574
1708 1582 1697 1574
1709 1582 1700 1574
1709 1582 1699 1571
1711 1583 1699 1571
1711 1584 1699 1575
1709 1572 1700 1568
1707 1580 1698 1572
1703 1573 1695 1568
C C C C C C
O C O C O C
1722 1588 1714 1566 1697 1556
1722 1590 1715 1567 1697 1557
1722 1588 1713 1564 1694 1557
1717 1587 1709 1564 1690 1555
1720 1586 1708 1563 1688 1557
1714 1587 1707 1564 1681 1555
1716 1587 1709 1564 1682 1556
1714 1587 1711 1563 1679 1554
1709 1586 1699 1563 1671 1554
1714 1586 1704 1563 1687 1554
1716 1584 1701 1563 1682 1555
C O C C
insb
insb
1716 1574
1710 1571
1711 1572
1711 1571
1713 1571
1712 1572
1702 1565
1708 1570
1703 1567
C C C C
1708 1579 1703 1565
1709 1581 1704 1566
1706 1576 1701 1560
1702 1572 1694 1557
1703 1571 1696 1557
1703 1577 1695 1559
1705 1573 1695 1558
1705 1572 1695 1558
1691 1566 1679 1553
1699 1571 1692 1555
1696 1567 1690 1552
O C O C
Solvent numeration is the same as in Table 1. Insoluble in this solvent.
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Table 3 The coefficients (s, a, b, and h) of the correlation equations ν˜ (C − X) = ν˜ (C X)0 + sπ* + aα + bβ + hδ2H , errors of the estimate (S.D.), Fisher indexes of reliability (F), and correlation coefficients (R) Enone
Conformer
ν˜ (C X)
ν˜ (C X)0 (cm−1 )
s
a
b
h
N
S.D.
F
R
EZE
C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
1694 1620 1682 1603 1672 1642 1663 1626 1722 1596 1708 1638 1698 1611 1716 1591 1707 NAb 1719 1589 1695 1566 1715 1555 1720 1578 1709 1575 1701 1564
−23.139 −4.459 −14.057 −5.454 −14.876 −11.427 −7.378 −2.812 −15.750 −12.195 −19.552 −9.396 −31.217 −15.741 −12.600 −9.008 −8.722 NA −12.242 −4.245 −12.843 −3.024 −13.883 −5.291 −16.944 −8.102 −12.421 −15.197 −18.680 −11.753
−3.964 −6.964 −5.600 −7.604 −4.957 −16.002 −7.149 −1.049 −2.303 NS NS NS NS −7.368 NS NS NS NA −9.991 NS −10.266 NS NS −4.003 NS NS NS NS −5.795 NS
8.728 −6.535 8.551 −3.410 NS −15.409 −4.341 −2.394 NSa NS NS NS NS NS NS NS −5.334 NA NS NS −3.960 NS NS NS −1.024 −2.453 NS −2.530 NS −2.935
6.075 −2.430 2.302 NS 5.597 7.958 NS NS 2.536 4.237 NS 2.412 3.560 7.481 NS −3.316 NS NA 3.821 NS 2.300 NS −1.030 2.406 0.357 0.48 NS 5.764 4.906 NS
11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 – 11 11 11 11 11 11 9 9 11 11 11 11
1.3005 1.5135 1.7006 1.2815 0.9413 6.3552 1.6161 0.8499 2.0045 0.9915 4.2908 1.1979 2.2911 2.3733 0.3769 1.1196 1.1489 – 1.1158 0.7053 1.6926 0.8781 0.9346 0.9747 0.5545 0.3645 0.6511 1.2443 0.4014 0.7827
56.553 42.372 22.905 39.853 51.768 6.2335 31.223 7.3724 10.628 23.267 5.2464 14.151 27.308 2.3185 277.45 34.173 27.936 – 20.601 9.8055 33.833 4.2284 47.599 2.0126 82.348 51.953 81.541 25.052 300.47 64.086
0.987 0.943 0.969 0.982 0.986 0.898 0.977 0.912 0.946 0.974 0.900 0.959 0.978 0.806 0.998 0.982 0.978 – 0.971 0.942 0.982 0.879 0.987 0.785 0.995 0.967 0.992 0.976 0.998 0.990
EZZ 1
EEE EEZ EZZ EEE
2
EEZ EZZ 3
EZE ZZZ EZZ
4
EZE EZZ
5
ZZZ 6
EZZ a b
O C O C O C O C O C O C O C O C O C O C O C O C O C O C O C
Eq. no. (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31)
Not significant. Not applicable.
of the estimate (S.D., the smaller the better), the Fisher index of reliability (F, the larger the better), and the correlation coefficient (R, variance = R2 , the closer to 1 the better). A correlation equation was considered as acceptable if the product correlation coefficient, R, indicates that the equation accounts for more than 80% of the variance (R2 ≥ 0.80). It is reasonable to consider the multiple correlations for enones 1–6 separately and then to compare the results obtained. Examination of Table 3 reveals some general features: for all stereoisomeric forms of studied enones statistically significant equations (R2 ≥ 0.80) are observed, the ν˜ (C C) band of the (EZE) stereoisomer of 3 being the exception as result of considerable overlapping of the ν˜ (C C) bands of the two stereoisomers [viz. (EZE) and (EZZ)] of the enone 3. Another similarity is that not all parameters of Eq. (2) are statistically significant. The enone wavenumbers are mainly related to the solvent’s dipolarity/polarizability parameter π* and to a much lesser extent to the solvent’s HBD acidity α, and to the solvent’s HBA basicity β.
of all four conformers exerts the dipolarity/polarizability term (π*): its regression coefficient s diminishes in absolute value almost synchronously with ν˜ (C O)0 (Fig. 1, line 䊉), showing that the lower the electron population of the carbonyl bond the smaller influence of the solvent’s dipolarity/polarizability (π*) is. Moreover, the coefficient s is negative, indicating ν˜ (C O) shift to lower wavenumbers with an increase of the solvent dipo-
3.1. Enone 1 Enone 1 equilibrates to four E-conformers (viz. EZE, EZZ, EEE and EEZ) with a percentage 43, 33, 10 and 14 mol.%, respectively. The largest influence on the ν˜ (C O) vibrations
Fig. 1. Plot of the coefficients s (䊉) and a () vs. ν˜ (C O)0 for MBO conformers (see Tables 2 and 3).
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larity/polarizability. Besides the dielectric effects (measured by π*), solvent molecules interact directly with the C O (and C C) group by two kinds of attractive forces. The first of these forces is that the solvent molecule forms a hydrogen bond with oxygen of the C O (or with system of this double bond). The capability of solvent to donate proton in a solvent-to-solute (enone) hydrogen bond is described by parameter α, solvent HBD acidity. The second is that the solvent molecule which has electronegative atoms O, N and S can form hydrogen bonds with a solute as proton donor. Parameter β describes the ability of a solvent to accept a proton in a solute-to-solvent hydrogen bond, i.e. its hydrogen bond acceptor capability with respect to a reference hydrogen bond donor. In enone 1 the formation of H-bond with vinylic ␣-hydrogen atom is possible owing to nucleophilic attack of solvent’s electronegative atoms. Solvent HBD acidity (α) has the perceptible effect on the ν˜ (C O) mode of (EZE), (EZZ) and (EEE) conformers. Increasing HBD acidity is associated with decreasing wavenumber. This is consistent with decreasing C O bond strength as modeled by the force constant, k, √ in the model for harmonic oscillator wavenumber, ν˜ = (2c)−1 kμ. Moreover, as can be seen from Fig. 1 (line ) the more polarizable carbonyl moiety is [the less wavenumber of ν˜ (C O) mode] the more significant is the solvent HBD acidity (α). From this point of view the (EEZ) conformer is the most suitable for hydrogen bond formation between the carbonyl group of the solute and a solvent molecule. This suggests these equations to be physically reasonable. From physical standpoint it should be expected that the coefficient b of the solvent HBA basicity (β) should be negative too, but in Eqs. (3) and (5) it is highly positive. The increase in wavenumber with increasing solvent HBA basicity suggests the physical reasonableness since the increase of oscillating mass shifts the ν˜ (C O) band to lower wavenumbers, therefore these positive values of b are apparent and are evidence for a lack of that kind of interaction with (EZE), (EZZ) and (EEE) conformers. In (EEZ) conformer the b value is negative and rather significant. The only possibility of the ν˜ (C O) shift to high wavenumbers with an increase of solvent HBA basicity (β) is the rotation of the carbonyl group out of the C C double bond plane (with disturbance of the conjugation between carbonyl and vinyl double bonds) as a result of the carbonyl specific solvation with solvent molecules, but at present we have no evidence to confirm this assumption. Contrary to the ν˜ (C O) the main influence on vibrations of the vinyl moiety makes the solvent HBD acidity (α) [a/s = 1.56 (EZE), Eq. (8); 1.39 (EZZ), Eq. (4); 1.40 (EZZ), Eq. (6)] with the exception of (EEZ) [a/b = 0.37, Eq. (8)] where the coefficient a is the smallest. Hence, the (EEZ) conformation is not convenient for a H-bond formation between the C C double bond and HBD solvents and the hydrogen bonding with these solvents takes place in the (EEZ) conformer presumably on the carbonyl moiety. All a values are negative, pointing out the ν˜ (C C) to decrease with increasing solvent HBD acidity (α). The same is true for s and b: they are negative too, so an increase of all three parameters, π*, α, and β, shifts the ν˜ (C C) to lower wavenumbers. A comparison of Eqs. (3)–(10) shows that the ν˜ (C O) band for all four conformers of MBO are shifted to lower wavenum-
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bers primarily due to dipole–dipole interactions with solvent molecules, while the ν˜ (C C) band shift to lower wavenumbers occurs to a greater extent due to the solvent HBD acidity (α). The vinylic vibrations are more sensitive to solvent HBD acidity (α) than that of ν˜ (C O) [with exception of the (EEZ) conformer]. 3.2. Enone 2 The replacement of the methyl group in 1 by CF3 in 2 increases the difference ˜ν between the ν˜ (C O) and the ν˜ (C C) band due to the high electron-withdrawing ability of the trifluoromethyl group. On the one hand, it increases the electron density at the C O double bond and hence shifts the ν˜ (C O) band to higher wavenumbers. On the other hand, on account of the higher electron withdrawal from the C C moiety by the trifluoromethyl carbonyl group, this produces a bathochromic shift of the ν˜ (C C) [14]. Simultaneously the ν˜ (C O) intensities A go down with an increase of the ν˜ (C C) intensities (in 1 the intensities A of the ν˜ (C O) and ν˜ (C C) are almost equal, whereas in 2 the A(C O) is 8–10-fold higher then the A(C C) [14,21]). Hence, the bond order of the C O double bond increases and that of the C C bond decreases as result of a weakening of the conjugation in the C C C O system [22]. For 2 there are three existing conformers of E-isomer (EZZ), (EEE) and (EEZ), with percentage 71, 23 and 6 mol.%, respectively [14]. The largest influence on ν˜ (C O) of all conformers has the solvent’s dipolarity/polarizability (π*): the higher the ν˜ (C O)0 of the conformers the higher is in absolute value the coefficient s (see Table 3). Solely for (EZZ), the coefficient a is retained [it is small and negative, Eq. (11)], while for other conformers it is not significant on the 0.95 significance level. The solvent HBA basicity term is insignificant for all three conformers. Similarly to the ν˜ (C O) absorption, the solvent dipolarity/polarizability (π*) term has the largest influence on the ν˜ (C C) absorption, whereas the solvent HBD acidity and HBA basicity terms are insignificant in Eqs. (12) and (14). Exception is Eq. (16) in which coefficient a is highly negative indicating that the hydrogen bond formation has appreciable effect on the ν˜ (C C) band of (EEZ). 3.3. Enone 3 Steric hindrance by the methyl group in -position of the 3 vinyl moiety reduces the number of conformers of E-isomer to two, namely (EZZ) and (EZE), the former being much more populated (92 mol.%). As can be easily seen from Table 3, solvent dipolarity/polarizability (π*) influences mainly on the stretching mode ν˜ (C O) of both conformers, while the solvent HBD acidity and HBA basicity terms are insignificant [Eqs. (20) and (22)] with exception of the (EZE) conformer, for which the solvent HBA basicity term is significant and highly negative. For the ν˜ (C C) band Eq. (1) is only applicable for the (EZZ) conformer. According to Eq. (18) in Table 3, the solvent dipolarity/polarizability (π*) term is highly negative, while other terms are insignificant. Hence, for the (EZZ) and (EZE) conformers of 3, like the ones of 2, the main contribution according to Eq. (1) makes the solvent dipolarity/polarizability (π*) term.
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3.4. Enone 4 When -CH3 is replaced by -C6 H5 the Z isomer [namely, the (ZZZ) conformer] appears in the enone 4 with appreciable percentage (35 mol.% in n-hexane) [14]. It is interesting to note that the coefficient s in Eqs. (20), (22), and (24) for the ν˜ (C O) band is highly negative and close to each other. Moreover, for the (ZZZ) and (EZZ) stereoisomeric forms the coefficient a is also highly negative, indicating the C O ability to form H-bonds with HBD solvents, while for (EZE) (Eq. (24)) this coefficient is insignificant. Hence, the bathochromic ν˜ (C O) band shift in the (ZZZ) and (EZZ) stereoisomers is mainly due to dipole–dipole interactions and H-bond formation with solvent molecules. The contribution of solvent basicity term (β) is insignificant in Eqs. (20)–(25), except in Eq. (22) where coefficient b is small in comparison with coefficients s and a. Hence, in both forms carbonyl and vinyl stretching vibrations are not susceptible to the influence of HBA solvents. 3.5. Enone 5 The strong electron-withdrawing nitro group in the 4-position of the benzene ring in 5 reduces a number of stereoisomers to only one, viz. (EZZ). A principal contribution to Eqs. (26) and (27) makes the solvent dipolarity/polarizability (π*) term, while neither the ν˜ (C O) nor the ν˜ (C C) vibrations are susceptible to solvent hydrogen bond acceptor basicity, differing from the corresponding conformer of 4 where the ν˜ (C O) vibration is highly sensitive to this term. Hence, the 4-NO2 C6 H4 -substituent, in contrast to C6 H5 -, decreases the negative charge at the oxygen atom of the carbonyl moiety, so it becomes unable to form hydrogen bonds with HBD solvents. 3.6. Enone 6 The distinguishing feature of this enone is the formation of two stereoisomeric forms (ZZZ) and (EZZ), the former being more populated (74 and 26% in n-hexane, respectively [14]). As in the enones described previously, the basic influence on the ν˜ (C O) and ν˜ (C C) band has the solvent polarity/polarizability (π*), whereas the solvent HBD acidity term (α) is insignificant [moderately negative for ν˜ (C O) of (EZZ) only, see (Eq. (30)]. From a comparison of Eqs. (20) and (21) and Eqs. (28) and (29) for the (ZZZ) stereoisomer of 3 and 5 one can conclude that the t-butyl group in -position of 6 prevents the carbonyl moiety to form H-bonds with solvent molecules, while the C O group of (EZZ) conformer forms hydrogen bonds with HBD solvents [a = −5.8, Eq. (30)]. The C C bond of the (EZZ) stereoisomer is shielded by the -substituent in both enones, 4 and 6. In contrast to the (ZZZ) stereoisomer of 6, the ν˜ (C O) band of (EZZ) form of 6, similar to the corresponding stereoisomeric form of 4, is shifted to the lower wavenumbers due to H-bond formation [cf. a in Eqs. (22) and (30)]. The coefficient b [Eqs. (29) and (31)] is significant for the ν˜ (C C) shift for both stereoisomers, indicating that the t-butyl substituent does not prevent from nucleophilic attack of basic solvent molecules on the positively charged -carbon of the C C double bond (or does not hinder in
H-bond formation between the solvent and vinylic ␣-hydrogen atom). Accordingly, a common feature of Eqs. (3)–(31) is the fact that dipolarity/polarizability plays the main role in the interactions of enones with solvent molecules, which leads to the observed bathochromic shifts of the ν˜ (C O) and ν˜ (C C) bands. The difference between the ν˜ (C O) band of solutes containing unconjugated carbonyl group and enones, where the C O double bond is conjugated with the vinyl moiety, is that the dominating solvent influence on former has the solvent HBD acidity [11], while for the enones solvent dipolarity/polarizability prevails. The insignificance of the solvent HBA basicity term for fluorinated enones is another general property of most equations listed in Table 3. In all enones (excepting 6) the conformer (EZZ) is most populated. Therefore, it is reasonable to compare the results obtained for this conformer of the enones examined. The trifluoromethyl group lowers the C O polarity, so the trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with HBD solvents (a/s = 0.40, 1 and 0.15, 2). Moreover, for ν˜ (C C) of the enone 1 the solvent HBD acidity (α) and the solvent HBA basicity term (β) play a perceptible role [a = −7.604, b = −3.410, Eq. (6)], while for 2 these terms are not significant (Eq. (12)). Hence, the system of the C C double bond in 2 is so polarized that it is unable to form hydrogen bonds with HBD solvents and the vinylic ␣-hydrogen atom does not participate in H-bonding with HBA solvent molecules. Based on the influence of -substituents on the ν˜ (C C) band shift one can divide -substituents into two kinds: -H, -C6 H5 , and -C(CH3 )3 assist in hydrogen bond formation with HBD solvents [coefficient a is negative and large enough, Eqs. (11), (22) and (30)], whereas -CH3 and -(4-NO2 C6 H4 ) prevent the C O group to form H-bonds with such solvents [the solvent HBD acidity term is not significant, Eqs. (17) and (26)]. The substituent -C6 H5 is a -donor which increases the negative charge on the carbon oxygen, thus promoting H-bond formation; -C(CH3 )3 interacts sterically with the carbonyl moiety of the trifluoroacetyl group, which rotates out of the C C double bond plane with weakening the C C C O conjugation and thus increasing the negative charge on carbonyl oxygen atom, as consequence. In the enone 1 all four existing conformers of the E-isomer are realized so it is desirable to compare the data for these conformers according to which EEE is the most polarizable, having high negative s, a, and b coefficients not only for the ν (C O) multiple linear regression but also for the same correlation of ν˜ (C C). It follows from Fig. 1 that the slopes of coefficients s and a are opposed to each other, hence the larger influence of the solvent dipolarity/polarizability has on ν˜ (C O) a smaller effect of the solvent HBD acidity on ν˜ (C O) is. There is no similar dependence for ν (C C), probably due to the existence of more than one center of H-bond formation. 4. Conclusion Contrary to simple carbonyl-containing molecules where the solvent HBD acidity has the largest influence on the ν˜ (C O) band shift to lower wavenumbers, the dipolarity/polarizability
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(π*) term plays the main role in the interactions of conjugated enones with solvent molecules and leads to bathochromic shift of the ν (C O) and ν (C C) bands. For all four conformers of 1 the ν˜ (C O) bands are shifted to the lower wavenumbers primarily due to dipole–dipole solute–solvent interactions, while the bathochromic ν˜ (C C) band shift occurs to a greater extent due to the solvent HBD acidity (α). The trifluoroacetyl group possesses a reduced ability to form hydrogen bonds with HBD solvents in comparison to the acetyl group. For the ν˜ (C C) band of the non-fluorinated enone 1 the solvent HBD acidity (α) and solvent HBA basicity term (β) play a perceptible role whereas for 2 these terms are not significant. -Substituents in fluorinated enones such as H, C6 H5 , and C(CH3 )3 assist in the hydrogen bond formation with HBD solvents, while -CH3 and 4-NO2 C6 H4 prevent the C O to form H-bonds with these solvents (the solvent HBD acidity term is not significant). The comparison of four conformers of the enone 1 reveals that (EEE) is most polarizable conformer. The influence of the solvent dipolarity/polarizability (π*) term and solvent HBD acidity (α) term on the bathochromic ν˜ (C O) band shift are opposite to each other. References [1] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 3rd ed., Wiley-VCH, 2003. [2] L.C.J. Almeida, P.S. Santos, Spectrochim. Acta: A 58 (2002) 3139.
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