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The nature and spectral effects of the interaction of certain polar solvents with benzophenone
JOURNAL
The
OF MOLECULAR
Nature Certain
3, l-16
SPECTROSCOPY
and
Spectral
Polar
Department
Effects
Solvents RSLPH
of Chemisty,
(1959)
with
of the Interaction
of
Benzophenone*
S. BECKER
Unil’ersity
of Houston,
Houston,
Tezas
An investigation was conducted to study the effect of certain solvent mixtures on the n --f rr* electronic absorption band and the fundamental carbonyl group vibration of benzophenone. The solvents principally investigated were nitriles, alcohol, and hydrocarbons. The nitrile-hydrocarbon-benzophenone mixtures exhibit an isobestic point indicative of a simple two species equilibrium. Also, for t,hese cases, the n - X* band maximum shifts 500 cm-1 toward higher frequency. A similar acetonitrile system exhibits no isobestic point but the n + X* maximum shifts 725 cm-’ toward higher frequency. Changes in the frequency of the fundamental carbonvl band in the infrared show a similar relation of effectiveness among the nitiiles. The data indicate electrostatic interaction between the positive C atom of the nitrile group and a nonbonding pair of electrons on the carbonyl oxygen. The complex is suggested to be of a monomer nitrile-benzophenone type except, in t’he case of the acetonitrile where a dimer nitrile-benzophenone complex also exists. The data on t,he effect of alcohol suggests a predominance ot’ monomer alcohol and dimer alcoholbenzophenone complexes. I. 1NTRc)DUCTION
McConnell (1) studied the effect of certain solvents on n --f ?r* band maxima. He ascribed the blue-shift to a stronger binding of polar solute molecules to the nonexcited molecules compared wit,h the excited molecules. Brealey and Kasha (2) studied the effect of ethanol on the ‘r~-+ x* band of benzophenone and pyridazine in the ultraviolet’ spectral region. With the aid of infrared data, they were able to show t,hat in the above solvents, the formation of a hydrogen bonded species was the principal reason for the 72+ ?r* band maximum being at shorter wavelength s. The origin of t,he so-called blue-shift was described in more specific, terms I ban that) done by RicConnell (1). The amount of the blue-shift was shown t,o be approximately equal to t,he energy involved in the hydrogen bonding. These aut,hors also poted that preliminary work indicated similar behavior for the system pyridazine-propionitrile* This research was xupport,ed in p:irt, by t.he Office of Naval Research, Project NRO19-617.
Physics
Branch,
2
BECKER
hexane but with less blue-shift. They ascribed the shift to a higher concentration of the nitrile around the pyridasine due to preferential orientation. Murray and Schneider (3) investigated the nature of the complexes formed between nitriles and CHC13 or hydrochloric acid by a study of binary freezing points. Acetonitrile in contrast to other nitriles showed no complexes with chloroform probably because of self association which apparently occurred only to a small extent or not at all in the higher homologues. The nitriles were considered to be electron donors contrary to the present investigation where the nitriles are considered as electron acceptors. This investigation was designed to (a) study the effects of active solvents as nitriles, chloroform and ethers on the 71 -+ ?r* transition in benzophenone, (b) determine t,he comparative strength of interaction of nitriles and other molecules such as alcohol, chloroform and ethers with benzophenone, (c) study of the effect of any interactions on the C=O and/or the C=N infrared bands, and (d) depending upon the effect of the nitriles and other molecules, determine the cause of the effect. II. EXPERIMENTAL
The principal technique employed was that of a progressive change in solvent medium from pure hydrocarbons (considered as inert solvents) to pure nitriles or other appropriate active solvents. In the ultraviolet region, this involved a study of the change caused by the varying solvent media on the position of the benzophenone n -+ ?r* maximum, the degree of change of the n -+ ?r* maximum and/or the formation of an isobestic point. In the infrared, a change in the frequency of the C=O and/or (34% bands or the presence of new bands in the same regions provided additional and unique evidence to determine the answers to the questions involved in this investigation. CHEMICALS AND SOLVENTS The benzophenone was Eastman White Label. Purification was accomplished by repeated recrystallization from alcohol. The ultraviolet and infrared spectrum showed the compound to be pure. An infrared spectrum comparison of recrystallized benzophenone with that taken directly from the bottle indicated no difference in purity. Thereafter, the benzophenone was used without further purification. All impure liquids were purified by fractional distillation through a 36-inch column packed with glass helices at reflux ratio of 12 to 1. Only middle fractions were collected. The n-propionitrile, n-butyronitrile, n-valeronitrile and benzonitrile were Eastman White Label. In each case the material was distilled over anhydrous CaS04. The boiling point range over which this material and all
INTERACTION OF POLAR SOLVENTS
3
others was collected was 0.5” C or less. Infrared analyses indicated the collected portion was pure. In addition, propionitrile, butyronitrile and valeronitrile were subjected to further analysis by gas-liquid partition chromatography. The instrument was a Burrell Kromo Tog, Model K-2. In at least one case, two different column packings were used to assure that the results were not dependent on the type column packing. In the above three cases, only one strong peak was observed indicating high purity. The acetonitrile used was Eastman spectrograde. It was taken from a freshly opened bot,tle and not purified further. The chloroform was Mallinckrodt, U. S. I’. This was purified by fractional distillation over anhydrous CaS& . The n-propyl ether was Eastman White Label. This was purified by fractional distillation over sodium metal. The ethyl alcohol was absolute pure ethyl alcohol from I?. S. Industrial Chemical Co. This was not further purified. Samples were taken from freshly opened bottles only. Inert solvents were hexane, cyclohexane, methyl cyclohexane, tetrachloroethylene and carbon tetrachloride. These were obtained either from Eastman or Phillips petroleum Special Products Division. All of these were spectrograde except for hexane. The hexane was purified by fractional distillat’ion over sodium in the manner as that described above. In addition, the hexane was passed slowly through activated silica gel, 60-200 mesh, obtained from the Davison Chemical Company. It should be noted that the care taken to assure purity is due t,o the nature of the problem. If a small quantity of any material, particularly water, is present which can interact with the benzophenone, other than the desired compound, false shifts would incur. INSTRUMENTS AND SPECTRA The instrument used for the ultraviolet portion was a Beckman Spectrophotometer Model DK-1 with automatic recording. The infrared studies were carried out using a Beckman Infrared Spectrophotometer Model IR-3, also with automatic recording. In most all cases except the chloroform and n-propyl ether, individual benzophenone samples were weighed for each percent run in the ultraviolet and infrared. All samples were stored in the dark at approximately 22” C. Benzophenone samples were weighed to an accuracy of f0.02 mg. The quantity of benaophenone used in each case was nearly the same. All pipettes and volumetric flasks were either factory retested or Xat~ional Bureau of Standards recalibrated. Due to a ?r + r* band adjacent t.o the benzophenone n + R* band, a correction was made in the degree of absorption of the n + ?r* in the region of the a + x* tail.
BECKER
4
III. RESULTS NITRILES
Ultraviolet The effect of solvent changes on the n + A* absorption band was studied by changing the solvent gradually from pure hydrocarbon to pure nitrile. There was one exception, this was for the case of acetonitrile where the solvent change was from tetrachloroethylene to pure acetonitrile. Acetonitrile is not soluble above approximately 1% by volume in hydrocarbons. The nitriles employed to study the change of the n -+ P* band were acetonitrile, propionitrile, butyronitrile, valeronitrile and benzonitrile. A typical result of this investigation is shown in Fig. 1. In all cases except acetonitrile, an isobestic point apparently exists. The locations of the isobestic points are as follows : propionitrile: butyronitrile valeronitrile: benzonitrile:
27,770-27,845 cm-‘, : 27,770-27,845 cm-l, 27,770-27,845 cm-‘, 27,655-27,731 cm-‘.
The indicated range is due to the fact that it is difficult to locate a true point with as many as seven curves passing through approximately the same location. The case of acetonitrile is somewhat different (Fig. 2). The most outstanding
0 24
25
26
27
28
29
WAVENUMBER-Ct.?X
30
31
32
33
IO-’
FIG. 1. Effect of valeronitrile on the TZ--f T* band of benzophenone. Concentrations of valeronitrile: 0, 1,5,20,50, and 100% by volume of a 4.96 X Wa M solution of benzophenone in cyclohexane.
INTERACTION
OF POLAR
SOLVENTS
5
features are: (a) a blue-shift of approximately 100 cm-’ of the n + ?r* maximum from hydrocarbon to tetrachloroethylene, (b) the vibration structure of benzophenone in tetrachloroethylene is not as defined as in a hydrocarbon (c) the 100% acetonitriIe curve on the low wavenumber side of the n + ?r* transition appears to be shifted to higher wavenumber compared to all other 100% nitrile curves and (d) an apparently unusual crossing of the 0, 1, and 5 % curves in the region of the minimum between the n + x* and ?r ---f ?r* bands. There is indication of an isobestic region in the vicinity of 27925 cm-l. The significance of these results will be discussed in Section IV. The structure of the n -+ T* benzophenone band is even less defined in carbon tetrachloride than in tetrachloroethylene. In all cases, extrapolation of the adjacent a + ?r* band tail had no effect on the isobestic points either regarding location or “sharpness”. All of the nitriles caused a shift of the n + s* maximum to higher wavenumber (blue-shift). The curves in Figs. 1 and 2 have not been corrected for absorption of the r + ?r* tail. The n + T* maximum in 100% nitrile is quite sensitive to the shape of the exponential curves used for extrapolation of the ?r + ?r* tail. Because of the difficulty sometimes encountered in matching the proper exponential curve to the ?r -+ A* tail, the n -+ X* maximum in 100 % has an accuracy of approximately &60 cm-‘. It should be noted that all energies have been obtained from original data in millimicrons. These statements also apply to the other molecules investigated herein: alcohol, chloroform, and n-propyl ether. Table I shows the 140
1
60. E 60.
24
25
26
27
2.9
29
WAVENUMBER-CM-’
30
31
32
33
X IO-’
FIG. 2. Effect of acetonitrile on the rz 4 T* band of benzophenone. Concentrations of acetonitrile: 0, 1,5,20,50, and 100% by volume of a 4.96 X 10-3 M solution of benaophenone in tetrachloroethylene.
6
BECKER T.4RIJ;
I
EFFECT0~ ACTIVESOLVENTSON 76--t 57* MAXIMUMS 0~ BESZOPIIEXONE Original 12-r* max. in hydrocarbon cm-’ CHaCN CH &H&N CH&HzCH&N CH&H~CHzCH~CW (GHdCN CH&H&H HCCIx
28,810 28,810 28,610 28,810 28,810 28,810 28,810
n-n’ max. tl.IO~;active solvent cm-’ 29,577 29,317 29,317 29,317 29,317 29,665 29,710
Corrected n-n* ma?t’ lOOT<, actwe solvent cm-’
Blue-shift cm-1
29,534 29,317 29,317 29,317 29,317 29,490 29,557
724 507 507 507 507 940 767
original location the n + ?r* maximum in hydrocarbon, the location of n + P* in 100 % nitrile uncorrected and corrected and the calculated blue-shift. Infrared
Figure 3 shows the effect of acetonitrile on the benzophenone C=C) stretch band at (1668.4 cm-l). Figure 3 reveals an apparent shift of the 1668 cm-l band to lower energy with increasing acetonitrile concentration. Further investigation at higher concent,rations of acetonitrile than shown in Fig. 3 indicates that the maximum shift of the 1668 cm-’ band has occurred by the time the concentration of acetonitrile is 25 % by volume. At higher concentration, no further shifting occurs. Additional study of the kUyronitrile and of the henzonitrile cases indicate much smaller shifts of the 1668 cm-’ band at concentrations comparable to that of acetonitrile. Table II summarizes the effect of nitriles on the 1668 cm+ band. All infrared data has a precision of fO.OO1 p (approximately 0.3 cm-‘). ALCOHOL
C’ltraviolet
Although a study of the effect of alcohol on benzophenone was not originally considered as part of this investigation, certain factors necessitated a reinvestigation of this case, particularly in the infrared, This combinat,ion has been previously investigated by Kasha and Brealey (2). The effect of varying percents of ethyl alcohol on the 7~-+ r* band of benzophenone was similar to that of Brealey and Kasha (2). In this case, the ?r + r* tail extrapolation has a slight effect on the optical density of the 100% alcohol curve in the region of the 27,616-28,003 cm-1 shoulder of the n + r* band. This is the region in which an isobestic point or region might be expected. Table I
INTERACTIOX
OF POLAR
SOLVENTS
100
1 so-
BO-
70.
20.
IO.
01
. 1620
,
,
,
,
1640
I660
1660
1700
WAVENUMBER
.
I
1720
CM-I
FIG. 3. The effect of acetonitrile on the C=O stretching band of benaophenone. Solutions of tetrachloroethylene and acetonitrile in a 0.1.mm KBr cell. Concentration of benzophenone, 0.20 M; acetonitrile, 5.0 M (26% by volume). Molar ratio, 25/l. Broken curve gives the C=O stretch band in tetrachloroethylene.
demonstrates the effect of alcohol on the YA-+ ?r* band maximum. This result is similar to that obtained by Brealey and Kasha. Infrared
Figures 4 and 5 demonstrate the effect of ethyl alcohol on the 1668 cm-l band of benzophenone. The principal points of interest are: (a) the presence of a band shoulder between 1663.4 cm-’ and 1656.3 cm-’ in Fig. 5 which is not present at the lower concentration, and (b) a shift of the 1668 cm-1 band to lower energy with increasing ethyl alcohol concentration. Table II summarizes the effect of alcohol on the 1668 cm-l band.
8
BECKER
TABLE; II EFFECT OF ACTIVE SOLVENTS ON THE 1668.cm-’ CARBONYL STRETCHBAND
-
Solvent concentration
BeW50-
T
Molar
Frequencyc=c concen- solvent bans;lulout tration .Obenzo Volume dolarity xlolarity phenom %
Solvent
6 26 50 75 5 37 1 10 0.5 30 50 95 100
Acetonitrile in tetrachloroethylene
Butyronitrile in cyclohexane Benzonitrile in cyclohexane Ethyl alcohol in tetrachloroethylene
CHLOROFORM
T
BENZOPHENONE
AND &?ROPYL
1shenone ratio
1.1 5.0 9.5 14.3 0.58 4.2 0.1 1.0 0.09 5.14 8.7 10.8 17.0
0.20 0.20 0.10 0.025 0.28 0.20 0.20 0.04 0.20 0.20 0.20 0.20 0.20
5.4/l 25/l 95/l 572/l 2/l 25/l 0.5/l 25/l 0.4/l 26/l 31/l 54/l 85/l
1668.4 cm-l 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4
Frequency.C=C) “2”:Ish
1668.1 cm-l 1663.7 1664 1663.7 1668.1 1667.3 1668.1 1667.3 1668.1 1666.7 1666.7 1666.7 1666.7
(-14 cm-1
0.3 4.7 4.4 4.7 0.3 1.1 0.3 1.1 0.3 1.7 1.7 1.7 1.7
ETHER
Ultraviolet Table I shows the effect of chloroform on the n -+ a* band maximum of benzophenone. In addition, there is indication of an isobestic point, (or perhaps region) in the vicinity of 27,690 cm-‘. Varying percents of n-propyl ether have little effect on the shape of the n + ?r* band or the position of its maximum. Essentially no blurring takes place even between the extremes of 0 % and 100 % propyl ether. The n + ?r* maximum blueshifts approximately 125 cm-’ in going from pure hydrocarbon to pure ether. IV. DISCUSSION
Ultraviolet 1. Propio-, Butyro-, Valero-, and Benzmitriles. With all of the nitriles except acetonitrile, mixtures of varying percents of nitriles and hydrocarbon with benzophenone result in isobestic points. The isobestic point of the individual nitriles occurs on the sharp shoulder of the benzophenone band in the region of 27,770 cm-‘. This indicates an equilibrium of two species. If only the propio- butyroand valeronitriles are considered, the nature of the species is questionable. One of the species is certainly benzophenone itself. For the other species, two possibilities seemingly exist: (a) that one of the hydrogens on the alkyl portion of the nitrile is protonic enough to form a weak hydrogen bond with the benzo-
INTERACTION
OF POLAR
SOLVENTS
100
90
60
,“‘1
1620
I
“I.“,”
1640
1660 WAVENUMBER
1680
1700
’
1720
CM-I
The effect of ethyl alcohol on the C==O stretching band of benzophenone. Solutions of tetrachloroethylene and ethyl alcohol in a O.l-mm KBr cell. Concentration of benzophenone, 0.20 M; ethyl alcohol 5.14 M (3070 by volume). Molar ratio, 26/l. Broken curve gives the C=O stretch band in tetrachloroethylene. FIG. 4.
phenone carbonyl oxygen and, (b) the carbon atom of the nitrile group is sufficiently positive to act as an electron acceptor of the nonbonding electrons of the carbonyl oxygen atom of benzophenone. However, when the case of benzonitrile is considered, one of these possibilities disappears. The benzonitrile molecule does not contain any hydrogen atom which could act protonic with a base as weak as benzophenone. Since an isobestic point is found for this case, the only apparent mode of interaction is a specific interaction between the positive C atom of the nitrile group and a nonbonding pair of electrons of the 0 atom of the carbonyl group of benzophenone. The above explanation is further substantiated by the degree of change of the n --, T* band maximum of benzophenone in a varying nitrile medium. The
10
BECKER
90
30
20
IO
0
FIG. 5. The effect of ethyl alcohol of the C=O stretching band of benzophenone in ethyl alcohol in a O.l-mm KBr cell. Concentration of benzophenone, 0.20 M; ethyl alcohol, 17.0 M (100% by volume). Molar ratio, 85/l. Broken curve gives the C=O stretch band in tetrachloroethylene.
n + P* band maximum in 100% propio-, butyro-, valero-, and benzonitrile is found approximately 500 cm-’ higher in energy than when in hydrocarbon. In view of the fact that benzonitrile seems to have only the one reasonable mode of interaction with benzophenone and that all of the other nitriles exhibit the same degree of energy change of the n + T* maximum as does benzonitrile, it would appear that a like mode of interaction is exhibited by all nitriles with benzophenone; that is, interaction of the positive C atom of the CN group of the nitrile and a pair of nonbonding electrons of the 0 atom of the C=O group of benzophenone. Further indication of the validity of this conclusion is found in the infrared studies. However, this argument will be considered in later discussion.
INTERACTIOK
OF
POLAR
SOLVENTS
11
The position of at least one vibrational band of the n + a* band of benzophenone seems unaffected by the varying solvent medium. The vibrational band in the vicinity of 26,400 cm-’ appears to maintain its position but gradually blurs as the percent of nitrile is increased (see Fig. 1). Similar results are noted for the shoulder band in the vicinity of 27,770 cm-‘. Thus, it appears that solvation effects (hereafter referred to as general solvation effects) other than those arising from specific interactions as discussed above are relatively inoperative
(4).
The mechanism proposed t,o explain the change of the n + ?r* band maximum (blue-shift) is similar to that for the case of hydrogen bonding (2). The formation of an electrostatic bond between the nonbonding electrons of the carbonyl oxygen and the positive C atom of the nitrile group lowers the energy of the n-electron orbital. Upon excitation, the electrostatic bond is either broken or severely weakened. The energy of the molecule in the excited state would be nearly equal to the energy of a molecule in the same excited state in which an electrostatic band has not previously existed. Thus, a blue-shift would occur. 2. Acetonitrile. The case of acetonitrile presents several unique features both in the ultraviolet region and infrared region. As previously noted, no isobestic point seems to exist for the acetonitrile-tetrachloroethylene-benzophenone system. In addition, other peculiarities exist such as the unusual curve crossing in the region of the a ---) H* tail as noted in the section on results. Furthermore, the change of the n --) ?r* band maximum in going from pure hydrocarbon to pure acetonitrile is larger (724 cm-l energy increase) for this syst,em than for any other nitrile system. At least some of these occurrences seems explicable in terms of a more positive nature of the C atom of the nitrile group of acetonitrile compared with the similar atom of the other nitriles. A more positive C atom would result in a stronger interaction with the nonbonding electrons of the 0 atom of benzophenone. Thus, the energy of the n-electron orbital with the interaction (in acetonitrile) would be lowered more than in the case of interaction with any other nitrile. Therefore, further displacement of the n ---) ?r* band maximum would result in the case of acetonitrile (for additional causes and explanation, see the section on infrared). The lack of an isobestic point is indirectly explained by the same reason as that used to account for the greater energy change of t,he n + ?r* band maximum by acetonitrile than by other nitriles. Investigations by Lambert et al. (5) and Murray and Schneider (3) indicate that acetonitrile undergoes self association. The former authors actually measured the heat of dimerization of acetonitrile in the vapor state. Thus, in the case of acetonitrile there would not be a simple two species equilibrium with benzophenone in contrast to the situation with the other nitriles and benzophenone. This would be manifested by the lack of an isobestic point for the benzophenone-acetonitrile-tetrachloroethylene system.
12
BECKER
Instead, an isobestic region would result indicative of several species in equilibrium (benzophenone, benzophenone-acetonitrile monomer, benzophenoneacetonitrile dimer and/or polymer). Despite these differences, it is expected that the mode of interaction of acetonitrile with benzophenone would be the same as that of the other nitriles. Although no direct evidence for lack of self association of other nitriles exists, the results herein discussed and the results of the invest,igation of Murray and Schneider (3) lend strong evidence supporting this claim. 8. n-Propyl Ether. The lack of significant change in the shape of the benzophenone n + 8 band and the position of its maximum in varying percents of n-propyl ether (including 100 %) indicate little interaction between benzophenone and n-propyl ether. This is not an unexpected result in view of the nature of the n-propyl ether molecule. 4. Chloroform. The n + I? benzophenone band maximum undergoes a rather large change (767 cm-l increase) in energy in going from pure hydrocarbon to pure chloroform. In addition, there is indication of an isobestic point or region for the benzophenone-chloroform-hydrocarbon system. The form of the interaction is very likely to be in the nature of hydrogen bonding. This provides additional evidence regarding the protonic nature of the chloroform H atom to that previously noted, for instance by Murray and Schneider (3). The degree of the change of the n + ?r* maximum indicates a similar strength of interaction of chloroform and acetonitrile with benzophenone. Moreover, their interaction ability is in a position intermediate to the other nitriles studied and alcohol with chloroform lying somewhat closer to alcohol than does acetonitrile. In view of the different mode of interaction of chloroform and acetonitrile, it is not surprising that chloroform would have a slightly stronger interaction with benzophenone than acetonitrile. Infrared 1. Alcohol. The information gained in the infrared studies of benzophenonetetrachloroethylene-alcohol system has bearing on the interpretation of the other results and therefore will be considered first. The 1668.4 cm-’ carbonyl fundamental band undergoes an apparent shift to lower energy with increasing alcohol concentration. However, by the time the alcohol/benzophenone molar ratio is 25/l, a maximum shift of 1.7 cm-’ has occurred. Thereafter, any increase of the alcohol/benzophenone ratio has no effect in shifting the 1668 cm-l band even up to 100 % alcohol (85/l). However, the above situation is complicated by the appearance of a new band shoulder in the 1663-1656 cm-’ region at an alcohol/benzophenone molar ratio of 25/l (see Fig. 4). This band shoulder was not present at a molar ratio of 0.4/l. At higher alcohol/benzophenone molar ratios, this shoulder becomes a peak. In 100% alcohol, the peak is at 1656.3 cm-’ (see Fig. 5). During the in-
INTERACTION
OF POLAR SOLVENTS
13
crease in the alcohol-benzophenone ratio, the 1668 cm-l band decreases in intensity while the 1656 cm-* band increases in intensity (see Figs. 4 and 5). One explanation consistent with above facts is the following. First, it is assumed that the apparent shifting of the 1668 cm-l band is due to the presence of a new band building up a lower energy. The new band being assigned as due to a benzophenone monomer alcohol complex. Thus, at a low molar alcohol/benzophenone (hereafter referred to as a/b) ratio of 0.4/l because of an excess of benzophenone and a low association constant for the complex (Z), the concentration of the monomer complex is low. Therefore, practically no shift of the 1668cm-’ band is observed. At the next a/b ratio studied 25/l, several changes have occurred. The 1668.4 cm-l band has undergone an apparent shift to 1666.7 (change of 1.7 cm-l) and a new shoulder, missing at the low a/b ratio, has attained considerable intensity. Meanwhile, the former 1668 cm-l band has decreased in intensity. The band at 1666.7 cm+ could be assigned as a new band due to the monomer complex or as the original 1668 cm+ band apparently shifted because of the build up of a new band at lower energy due to monomer complex formation. The shoulder band at 1656 cm-l is assigned to a benzophenonealcohol dimer and/or benzophenone-alcohol polymer complex. Thus, the 1666.7cm-1 band is assigned to a benzophenone-complex monomer complex. The absence of any shoulder at a low alcohol concentration and low a/b ratio and the relative intensity changes of the bands would tend to substantiate the above conclusions. At a low alcohol concentration (0.08 M) and low a/b ratio (0.4/l) the complex formed would most likely be monomeric in nature and little self association of alcohol would take place. At a higher alcohol concentration (5.1 M) and a/b ratio (25/l), alcohol dimer and/or alcohol polymer type complexes would form, thus lowering the relative concentration of the benzophenone-alcohol monomer type as is reflected in the band intensity relationships. By the time the concentration of alcohol is even higher (100 % by volume or 17 M) where the a/b ratio is 85/l, the shoulder has become a band at 1656 cm-1 with further increase in int)ensity and accompanying intensity lowering of the 1666 cm-l benzophenone-alcohol monomer complex band. The situation is essentially the same for alcohol at a concentration of 95% where the a/b ratio is 54/l. These results would be in accord with the explanation in the preceding paragraphs. The difference between the 1668 cm-l carbonyl band (in tetrachloroethylene) and the 1656 cm-1 carbonyl band (in alcohol) is approximately 12 cm-l. This difference is similar to carbonyl frequency shifts obtained by other investigators (6’,7). A general literature search on the problem of the effect of alcoholic hydrogen bonding on the C=O group frequency indicated that the circumstance of two bands of nearly equal intensity in the C=O group frequency region did not
14
BECKER
occur. This situation could arise because of resolution problems and/or the IXture of the solvent-solute, and/or the solvent-solvent interactions. Thus, if no strong solvent-solvent interaction existed only one band would exist, that of the solute-solvent monomer complex. If on the other hand strong solvent-solvent interaction was possible, only one carbonyl band would exist, that of a solutesolvent polymer complex. If bands due to both solute-monomer solvent and solute-polymer solvent did exist, poor resolution would not indicate the true fact. Further consideration of the nature of the benzophenone-alcohol complexes that could be formed indicate the proper relative positions of the monomer and polymer assigned bonds. The complexing of one alcohol molecule to a benzophenone molecule would tend to lower the frequency of the carbonyl group band. This would result because of an inductive effect caused by the protonic aIcoholic hydrogen atom. The formation of alcohol dimer types complexes of the form \ /
C=O---H-0-CH2--CH3 : H-0-CHZ-CH,
tend to further increase the protonic nature of the keto bonded alcoholic hydrogen atom. Therefore, the carbonyl bonding electrons are subjected to a greater inductive pull than before causing a further change (decrease) of the carbonyl band frequency. The formation of higher order alcohol complexes should have only relatively minor added effects such as a band broadening effect. Consequently, in essence only two distinct bands would arise. In addition, the effect of a monomer solvation process on the carbonyl band frequency would appear to be less than that of a dimer type solvation process. Further, the dimer type solvation process would occur at only relatively high alcohol concentration. All of these facts are consistent with the intensity changes, intensity relations and relative positions of the bands found in this investigation. It is interesting to note that the explanation, as given above, indicates a fairly appreciable concentration of benzophenone-alcohol monomer complex exists even in pure alcohol. Finally, it should be noted that an alternative explanation could exist. This would assign the 1656 cm-’ band as due to monomer solvation. Thus, the 1666 cm-’ band would be the unperturbed carbonyl frequency (at 1668 cm-l) shifted by a general solvation effect.. The principal problems with this explanation are (a) considering the intensity of the 1666 cm-’ band, it seems unreasonable that uncomplexed benzophenone of the concentration indicated by this band could exist in 100% alcohol, (b) the fact that the 1668 cm-l band although shifting to 1666 cm-’ does not continue to shift upon further increase of the eth-
INTERACTION
OF POLAR
SOLVENTS
15
anol concentration as might be expected for a general solvation effect if it existed. In addition, it should be remembered that this investigation and that of Brealey and Kasha (2) indicated the apparent absence of solvation effects other than those of a specific nature as described herein. 9. Acetonitrile. In the case of acetonitrile, the 1668.4 cm-’ carbonyl band undergoes an apparent shift to lower energy with increasing acetonitrile concentration. By the time a nitrile/benzophenone molar ratio of 25/l is attained, the shift has reached its maximum value of 4.5 cm-l. Thereafter, there is no further shift of the band with increasing concentration up to a nitrilelbenzophenone ratio of 572/l. This shift is accompanied by a small decrease in intensity of the band as well as a small band broadening. The apparent shift of the band is not likely to be due to a general solvation effect since the shift ceases even though the concentration of acetonitrile is further increased by a factor of 3. The nitrile/benzophenone molar ratio is increased by a factor of 23 (see Table II). Although it might be that the shift would be predicted to cease because of the nature of the equations describing general solvation shifts, it does not seem plausible that the effect would be noticeable at the lower of the concentrations studies. Thus, the apparent shift is probably due to the fact that new bands have grown in at lower energy causing the apparent shift. The facts are in accord with the view that new species exist which have altered the carbonyl frequency. The nature of these species have been discussed in the ultraviolet section of Section IV. 3. Butyro- and Benzonitrile. The addition of butyro- or benzonitrile to benzophenone also causes an apparent shift of the 1668 cm-’ carbonyl band t’o lower energy. Here as in the cases of acetonitrile and alcohol, the shift reaches a maximum at a nitrile/benzophenone molar ratio of 25/l. The degree of the shift is 1.2 cm-‘. The cause of this shift would be the same as that for acetonitrile except that only a single new specie exists, that of a monomer nitrile-benzophenone complex. The fact that the shift is much less for these cases than for acetonitrile could be due to one or both of the following: (a) the nitrile-benzophenone complex formed is less stable than that formed by the acetonitrile, (b) since acetonitrile can dimerize, and additional dimer-type complex is formed whose band would appear at lower energy than the monomeric type. This would be similar to the case of alcohol where the dimer-type complex is shifted further to lower energy than is the monomer. Aeetonitrile can dimerize as indicated by other work (3,5) and the ultraviolet section of this investigation. The other nitriles seem incapable of dimerization as indicated by others (3) and the formation of a simple isobestic point, in the ultraviolet section of this work. The principal reason for the greater apparent change in the 1668 cm-’ band
BECKER
16
for the case of acetonitrile would appear to be due to the formation of an additional dimer-type complex of the acetonitrile-benzophenone. The dimer-type complex intrinsically signifies a complex of greater strength than the monomer type. This then would account for the apparent shift of the 1668 cm-’ band being (a) larger than for the other nitriles, (b) larger than for the alcohol-monomer type complex and (c) less than, but more nearly like that of the band due to t’he alcohol-dimer-type complex. Because of the difference in nature of interaction, it would be expected that dimer-alcohol-benzophenone complexes would produce greater inductive effects on the carbonyl group electrons than dimer acetonitrilebenzophenone complexes. This would result in a larger change of the carbonyl band frequency for the alcohol-type complex. Both the ultraviolet and infrared investigations indicate similar relative degrees of interaction of nitriles and alcohol with benzophenone. Thus, acetonitrile lies between the other nitriles and alcohol in affecting the change of the n -+ ?r* maximum to the blue. The greater change of the n + ?r* maximum caused by acetonitrile compared with other nitriles would appear in part to be due to the formation of an additional dimer-type acetonitrile-benzophenone complex. The justification of this conclusion would be based on arguments similar to those employed to explain the like phenomenon occurring for the carbonyl band. 4. E$ects on the Nitrile Group Frequency. The effect of the interaction of aceto-, butyro- and benzonitriles with benzophenone on the nitrile group band in the 2272 cm-1 region was examined. There was no notable shift of the nitrile group frequency upon interaction with benzophenone for any of these nitriles. In at least one instance, acetonitrile, studies were made up to a nitrile concentration of 14.3 M which corresponded to a nitrile/benzophenone molar ratio of 572/l. In this latter case, there was indication of a new band(s) at higher frequency, ACKNOWLEDGMENT
to Dr. Jose Fernandez and Dr. Eion McRae for helpful discussions regarding the results of this investigation. Also, to Mrs. Jean B. Allison for her help in obtaining some of the spectral data. The author
RECEIVED:
wishes to express
March
appreciation
31, 1958 REFERENCES
1. H. MCCONNELL, J. Chem. Phys. 20, 700 (1952). 2. G. J. BREALEY AND M. KASHA, J. Am. Chem. Sot. 77.4462 (1955). 8. F. E. MURRAY AND W. G. SCHNEIDER, Can. J. Chem. 33, 797 (1955). 4. E. G. MCRAE, J. Phys. Chem. 61, 562 (1957). 6. J. D. LAMBERT, G. A. ROBERTS, J. S. ROWLINSON, AND V. J. WILKINSON, Proc. Roy. Sot. A196, 113 (1949). 6. R. S. RASMUSSEN, D. D. TUNNICLIFF, AND R. R. BRATTAIN, J. Am. Chem. Sot. 71, 1068 (1949). 7. W. GORDY, J. Am. Chem. Sot. 60, 605 (1938).
The
OF MOLECULAR
Nature Certain
3, l-16
SPECTROSCOPY
and
Spectral
Polar
Department
Effects
Solvents RSLPH
of Chemisty,
(1959)
with
of the Interaction
of
Benzophenone*
S. BECKER
Unil’ersity
of Houston,
Houston,
Tezas
An investigation was conducted to study the effect of certain solvent mixtures on the n --f rr* electronic absorption band and the fundamental carbonyl group vibration of benzophenone. The solvents principally investigated were nitriles, alcohol, and hydrocarbons. The nitrile-hydrocarbon-benzophenone mixtures exhibit an isobestic point indicative of a simple two species equilibrium. Also, for t,hese cases, the n - X* band maximum shifts 500 cm-1 toward higher frequency. A similar acetonitrile system exhibits no isobestic point but the n + X* maximum shifts 725 cm-’ toward higher frequency. Changes in the frequency of the fundamental carbonvl band in the infrared show a similar relation of effectiveness among the nitiiles. The data indicate electrostatic interaction between the positive C atom of the nitrile group and a nonbonding pair of electrons on the carbonyl oxygen. The complex is suggested to be of a monomer nitrile-benzophenone type except, in t’he case of the acetonitrile where a dimer nitrile-benzophenone complex also exists. The data on t,he effect of alcohol suggests a predominance ot’ monomer alcohol and dimer alcoholbenzophenone complexes. I. 1NTRc)DUCTION
McConnell (1) studied the effect of certain solvents on n --f ?r* band maxima. He ascribed the blue-shift to a stronger binding of polar solute molecules to the nonexcited molecules compared wit,h the excited molecules. Brealey and Kasha (2) studied the effect of ethanol on the ‘r~-+ x* band of benzophenone and pyridazine in the ultraviolet’ spectral region. With the aid of infrared data, they were able to show t,hat in the above solvents, the formation of a hydrogen bonded species was the principal reason for the 72+ ?r* band maximum being at shorter wavelength s. The origin of t,he so-called blue-shift was described in more specific, terms I ban that) done by RicConnell (1). The amount of the blue-shift was shown t,o be approximately equal to t,he energy involved in the hydrogen bonding. These aut,hors also poted that preliminary work indicated similar behavior for the system pyridazine-propionitrile* This research was xupport,ed in p:irt, by t.he Office of Naval Research, Project NRO19-617.
Physics
Branch,
2
BECKER
hexane but with less blue-shift. They ascribed the shift to a higher concentration of the nitrile around the pyridasine due to preferential orientation. Murray and Schneider (3) investigated the nature of the complexes formed between nitriles and CHC13 or hydrochloric acid by a study of binary freezing points. Acetonitrile in contrast to other nitriles showed no complexes with chloroform probably because of self association which apparently occurred only to a small extent or not at all in the higher homologues. The nitriles were considered to be electron donors contrary to the present investigation where the nitriles are considered as electron acceptors. This investigation was designed to (a) study the effects of active solvents as nitriles, chloroform and ethers on the 71 -+ ?r* transition in benzophenone, (b) determine t,he comparative strength of interaction of nitriles and other molecules such as alcohol, chloroform and ethers with benzophenone, (c) study of the effect of any interactions on the C=O and/or the C=N infrared bands, and (d) depending upon the effect of the nitriles and other molecules, determine the cause of the effect. II. EXPERIMENTAL
The principal technique employed was that of a progressive change in solvent medium from pure hydrocarbons (considered as inert solvents) to pure nitriles or other appropriate active solvents. In the ultraviolet region, this involved a study of the change caused by the varying solvent media on the position of the benzophenone n -+ ?r* maximum, the degree of change of the n -+ ?r* maximum and/or the formation of an isobestic point. In the infrared, a change in the frequency of the C=O and/or (34% bands or the presence of new bands in the same regions provided additional and unique evidence to determine the answers to the questions involved in this investigation. CHEMICALS AND SOLVENTS The benzophenone was Eastman White Label. Purification was accomplished by repeated recrystallization from alcohol. The ultraviolet and infrared spectrum showed the compound to be pure. An infrared spectrum comparison of recrystallized benzophenone with that taken directly from the bottle indicated no difference in purity. Thereafter, the benzophenone was used without further purification. All impure liquids were purified by fractional distillation through a 36-inch column packed with glass helices at reflux ratio of 12 to 1. Only middle fractions were collected. The n-propionitrile, n-butyronitrile, n-valeronitrile and benzonitrile were Eastman White Label. In each case the material was distilled over anhydrous CaS04. The boiling point range over which this material and all
INTERACTION OF POLAR SOLVENTS
3
others was collected was 0.5” C or less. Infrared analyses indicated the collected portion was pure. In addition, propionitrile, butyronitrile and valeronitrile were subjected to further analysis by gas-liquid partition chromatography. The instrument was a Burrell Kromo Tog, Model K-2. In at least one case, two different column packings were used to assure that the results were not dependent on the type column packing. In the above three cases, only one strong peak was observed indicating high purity. The acetonitrile used was Eastman spectrograde. It was taken from a freshly opened bot,tle and not purified further. The chloroform was Mallinckrodt, U. S. I’. This was purified by fractional distillation over anhydrous CaS& . The n-propyl ether was Eastman White Label. This was purified by fractional distillation over sodium metal. The ethyl alcohol was absolute pure ethyl alcohol from I?. S. Industrial Chemical Co. This was not further purified. Samples were taken from freshly opened bottles only. Inert solvents were hexane, cyclohexane, methyl cyclohexane, tetrachloroethylene and carbon tetrachloride. These were obtained either from Eastman or Phillips petroleum Special Products Division. All of these were spectrograde except for hexane. The hexane was purified by fractional distillat’ion over sodium in the manner as that described above. In addition, the hexane was passed slowly through activated silica gel, 60-200 mesh, obtained from the Davison Chemical Company. It should be noted that the care taken to assure purity is due t,o the nature of the problem. If a small quantity of any material, particularly water, is present which can interact with the benzophenone, other than the desired compound, false shifts would incur. INSTRUMENTS AND SPECTRA The instrument used for the ultraviolet portion was a Beckman Spectrophotometer Model DK-1 with automatic recording. The infrared studies were carried out using a Beckman Infrared Spectrophotometer Model IR-3, also with automatic recording. In most all cases except the chloroform and n-propyl ether, individual benzophenone samples were weighed for each percent run in the ultraviolet and infrared. All samples were stored in the dark at approximately 22” C. Benzophenone samples were weighed to an accuracy of f0.02 mg. The quantity of benaophenone used in each case was nearly the same. All pipettes and volumetric flasks were either factory retested or Xat~ional Bureau of Standards recalibrated. Due to a ?r + r* band adjacent t.o the benzophenone n + R* band, a correction was made in the degree of absorption of the n + ?r* in the region of the a + x* tail.
BECKER
4
III. RESULTS NITRILES
Ultraviolet The effect of solvent changes on the n + A* absorption band was studied by changing the solvent gradually from pure hydrocarbon to pure nitrile. There was one exception, this was for the case of acetonitrile where the solvent change was from tetrachloroethylene to pure acetonitrile. Acetonitrile is not soluble above approximately 1% by volume in hydrocarbons. The nitriles employed to study the change of the n -+ P* band were acetonitrile, propionitrile, butyronitrile, valeronitrile and benzonitrile. A typical result of this investigation is shown in Fig. 1. In all cases except acetonitrile, an isobestic point apparently exists. The locations of the isobestic points are as follows : propionitrile: butyronitrile valeronitrile: benzonitrile:
27,770-27,845 cm-‘, : 27,770-27,845 cm-l, 27,770-27,845 cm-‘, 27,655-27,731 cm-‘.
The indicated range is due to the fact that it is difficult to locate a true point with as many as seven curves passing through approximately the same location. The case of acetonitrile is somewhat different (Fig. 2). The most outstanding
0 24
25
26
27
28
29
WAVENUMBER-Ct.?X
30
31
32
33
IO-’
FIG. 1. Effect of valeronitrile on the TZ--f T* band of benzophenone. Concentrations of valeronitrile: 0, 1,5,20,50, and 100% by volume of a 4.96 X Wa M solution of benzophenone in cyclohexane.
INTERACTION
OF POLAR
SOLVENTS
5
features are: (a) a blue-shift of approximately 100 cm-’ of the n + ?r* maximum from hydrocarbon to tetrachloroethylene, (b) the vibration structure of benzophenone in tetrachloroethylene is not as defined as in a hydrocarbon (c) the 100% acetonitriIe curve on the low wavenumber side of the n + ?r* transition appears to be shifted to higher wavenumber compared to all other 100% nitrile curves and (d) an apparently unusual crossing of the 0, 1, and 5 % curves in the region of the minimum between the n + x* and ?r ---f ?r* bands. There is indication of an isobestic region in the vicinity of 27925 cm-l. The significance of these results will be discussed in Section IV. The structure of the n -+ T* benzophenone band is even less defined in carbon tetrachloride than in tetrachloroethylene. In all cases, extrapolation of the adjacent a + ?r* band tail had no effect on the isobestic points either regarding location or “sharpness”. All of the nitriles caused a shift of the n + s* maximum to higher wavenumber (blue-shift). The curves in Figs. 1 and 2 have not been corrected for absorption of the r + ?r* tail. The n + T* maximum in 100% nitrile is quite sensitive to the shape of the exponential curves used for extrapolation of the ?r + ?r* tail. Because of the difficulty sometimes encountered in matching the proper exponential curve to the ?r -+ A* tail, the n -+ X* maximum in 100 % has an accuracy of approximately &60 cm-‘. It should be noted that all energies have been obtained from original data in millimicrons. These statements also apply to the other molecules investigated herein: alcohol, chloroform, and n-propyl ether. Table I shows the 140
1
60. E 60.
24
25
26
27
2.9
29
WAVENUMBER-CM-’
30
31
32
33
X IO-’
FIG. 2. Effect of acetonitrile on the rz 4 T* band of benzophenone. Concentrations of acetonitrile: 0, 1,5,20,50, and 100% by volume of a 4.96 X 10-3 M solution of benaophenone in tetrachloroethylene.
6
BECKER T.4RIJ;
I
EFFECT0~ ACTIVESOLVENTSON 76--t 57* MAXIMUMS 0~ BESZOPIIEXONE Original 12-r* max. in hydrocarbon cm-’ CHaCN CH &H&N CH&HzCH&N CH&H~CHzCH~CW (GHdCN CH&H&H HCCIx
28,810 28,810 28,610 28,810 28,810 28,810 28,810
n-n’ max. tl.IO~;active solvent cm-’ 29,577 29,317 29,317 29,317 29,317 29,665 29,710
Corrected n-n* ma?t’ lOOT<, actwe solvent cm-’
Blue-shift cm-1
29,534 29,317 29,317 29,317 29,317 29,490 29,557
724 507 507 507 507 940 767
original location the n + ?r* maximum in hydrocarbon, the location of n + P* in 100 % nitrile uncorrected and corrected and the calculated blue-shift. Infrared
Figure 3 shows the effect of acetonitrile on the benzophenone C=C) stretch band at (1668.4 cm-l). Figure 3 reveals an apparent shift of the 1668 cm-l band to lower energy with increasing acetonitrile concentration. Further investigation at higher concent,rations of acetonitrile than shown in Fig. 3 indicates that the maximum shift of the 1668 cm-’ band has occurred by the time the concentration of acetonitrile is 25 % by volume. At higher concentration, no further shifting occurs. Additional study of the kUyronitrile and of the henzonitrile cases indicate much smaller shifts of the 1668 cm-’ band at concentrations comparable to that of acetonitrile. Table II summarizes the effect of nitriles on the 1668 cm+ band. All infrared data has a precision of fO.OO1 p (approximately 0.3 cm-‘). ALCOHOL
C’ltraviolet
Although a study of the effect of alcohol on benzophenone was not originally considered as part of this investigation, certain factors necessitated a reinvestigation of this case, particularly in the infrared, This combinat,ion has been previously investigated by Kasha and Brealey (2). The effect of varying percents of ethyl alcohol on the 7~-+ r* band of benzophenone was similar to that of Brealey and Kasha (2). In this case, the ?r + r* tail extrapolation has a slight effect on the optical density of the 100% alcohol curve in the region of the 27,616-28,003 cm-1 shoulder of the n + r* band. This is the region in which an isobestic point or region might be expected. Table I
INTERACTIOX
OF POLAR
SOLVENTS
100
1 so-
BO-
70.
20.
IO.
01
. 1620
,
,
,
,
1640
I660
1660
1700
WAVENUMBER
.
I
1720
CM-I
FIG. 3. The effect of acetonitrile on the C=O stretching band of benaophenone. Solutions of tetrachloroethylene and acetonitrile in a 0.1.mm KBr cell. Concentration of benzophenone, 0.20 M; acetonitrile, 5.0 M (26% by volume). Molar ratio, 25/l. Broken curve gives the C=O stretch band in tetrachloroethylene.
demonstrates the effect of alcohol on the YA-+ ?r* band maximum. This result is similar to that obtained by Brealey and Kasha. Infrared
Figures 4 and 5 demonstrate the effect of ethyl alcohol on the 1668 cm-l band of benzophenone. The principal points of interest are: (a) the presence of a band shoulder between 1663.4 cm-’ and 1656.3 cm-’ in Fig. 5 which is not present at the lower concentration, and (b) a shift of the 1668 cm-1 band to lower energy with increasing ethyl alcohol concentration. Table II summarizes the effect of alcohol on the 1668 cm-l band.
8
BECKER
TABLE; II EFFECT OF ACTIVE SOLVENTS ON THE 1668.cm-’ CARBONYL STRETCHBAND
-
Solvent concentration
BeW50-
T
Molar
Frequencyc=c concen- solvent bans;lulout tration .Obenzo Volume dolarity xlolarity phenom %
Solvent
6 26 50 75 5 37 1 10 0.5 30 50 95 100
Acetonitrile in tetrachloroethylene
Butyronitrile in cyclohexane Benzonitrile in cyclohexane Ethyl alcohol in tetrachloroethylene
CHLOROFORM
T
BENZOPHENONE
AND &?ROPYL
1shenone ratio
1.1 5.0 9.5 14.3 0.58 4.2 0.1 1.0 0.09 5.14 8.7 10.8 17.0
0.20 0.20 0.10 0.025 0.28 0.20 0.20 0.04 0.20 0.20 0.20 0.20 0.20
5.4/l 25/l 95/l 572/l 2/l 25/l 0.5/l 25/l 0.4/l 26/l 31/l 54/l 85/l
1668.4 cm-l 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4 1668.4
Frequency.C=C) “2”:Ish
1668.1 cm-l 1663.7 1664 1663.7 1668.1 1667.3 1668.1 1667.3 1668.1 1666.7 1666.7 1666.7 1666.7
(-14 cm-1
0.3 4.7 4.4 4.7 0.3 1.1 0.3 1.1 0.3 1.7 1.7 1.7 1.7
ETHER
Ultraviolet Table I shows the effect of chloroform on the n -+ a* band maximum of benzophenone. In addition, there is indication of an isobestic point, (or perhaps region) in the vicinity of 27,690 cm-‘. Varying percents of n-propyl ether have little effect on the shape of the n + ?r* band or the position of its maximum. Essentially no blurring takes place even between the extremes of 0 % and 100 % propyl ether. The n + ?r* maximum blueshifts approximately 125 cm-’ in going from pure hydrocarbon to pure ether. IV. DISCUSSION
Ultraviolet 1. Propio-, Butyro-, Valero-, and Benzmitriles. With all of the nitriles except acetonitrile, mixtures of varying percents of nitriles and hydrocarbon with benzophenone result in isobestic points. The isobestic point of the individual nitriles occurs on the sharp shoulder of the benzophenone band in the region of 27,770 cm-‘. This indicates an equilibrium of two species. If only the propio- butyroand valeronitriles are considered, the nature of the species is questionable. One of the species is certainly benzophenone itself. For the other species, two possibilities seemingly exist: (a) that one of the hydrogens on the alkyl portion of the nitrile is protonic enough to form a weak hydrogen bond with the benzo-
INTERACTION
OF POLAR
SOLVENTS
100
90
60
,“‘1
1620
I
“I.“,”
1640
1660 WAVENUMBER
1680
1700
’
1720
CM-I
The effect of ethyl alcohol on the C==O stretching band of benzophenone. Solutions of tetrachloroethylene and ethyl alcohol in a O.l-mm KBr cell. Concentration of benzophenone, 0.20 M; ethyl alcohol 5.14 M (3070 by volume). Molar ratio, 26/l. Broken curve gives the C=O stretch band in tetrachloroethylene. FIG. 4.
phenone carbonyl oxygen and, (b) the carbon atom of the nitrile group is sufficiently positive to act as an electron acceptor of the nonbonding electrons of the carbonyl oxygen atom of benzophenone. However, when the case of benzonitrile is considered, one of these possibilities disappears. The benzonitrile molecule does not contain any hydrogen atom which could act protonic with a base as weak as benzophenone. Since an isobestic point is found for this case, the only apparent mode of interaction is a specific interaction between the positive C atom of the nitrile group and a nonbonding pair of electrons of the 0 atom of the carbonyl group of benzophenone. The above explanation is further substantiated by the degree of change of the n --, T* band maximum of benzophenone in a varying nitrile medium. The
10
BECKER
90
30
20
IO
0
FIG. 5. The effect of ethyl alcohol of the C=O stretching band of benzophenone in ethyl alcohol in a O.l-mm KBr cell. Concentration of benzophenone, 0.20 M; ethyl alcohol, 17.0 M (100% by volume). Molar ratio, 85/l. Broken curve gives the C=O stretch band in tetrachloroethylene.
n + P* band maximum in 100% propio-, butyro-, valero-, and benzonitrile is found approximately 500 cm-’ higher in energy than when in hydrocarbon. In view of the fact that benzonitrile seems to have only the one reasonable mode of interaction with benzophenone and that all of the other nitriles exhibit the same degree of energy change of the n + T* maximum as does benzonitrile, it would appear that a like mode of interaction is exhibited by all nitriles with benzophenone; that is, interaction of the positive C atom of the CN group of the nitrile and a pair of nonbonding electrons of the 0 atom of the C=O group of benzophenone. Further indication of the validity of this conclusion is found in the infrared studies. However, this argument will be considered in later discussion.
INTERACTIOK
OF
POLAR
SOLVENTS
11
The position of at least one vibrational band of the n + a* band of benzophenone seems unaffected by the varying solvent medium. The vibrational band in the vicinity of 26,400 cm-’ appears to maintain its position but gradually blurs as the percent of nitrile is increased (see Fig. 1). Similar results are noted for the shoulder band in the vicinity of 27,770 cm-‘. Thus, it appears that solvation effects (hereafter referred to as general solvation effects) other than those arising from specific interactions as discussed above are relatively inoperative
(4).
The mechanism proposed t,o explain the change of the n + ?r* band maximum (blue-shift) is similar to that for the case of hydrogen bonding (2). The formation of an electrostatic bond between the nonbonding electrons of the carbonyl oxygen and the positive C atom of the nitrile group lowers the energy of the n-electron orbital. Upon excitation, the electrostatic bond is either broken or severely weakened. The energy of the molecule in the excited state would be nearly equal to the energy of a molecule in the same excited state in which an electrostatic band has not previously existed. Thus, a blue-shift would occur. 2. Acetonitrile. The case of acetonitrile presents several unique features both in the ultraviolet region and infrared region. As previously noted, no isobestic point seems to exist for the acetonitrile-tetrachloroethylene-benzophenone system. In addition, other peculiarities exist such as the unusual curve crossing in the region of the a ---) H* tail as noted in the section on results. Furthermore, the change of the n --) ?r* band maximum in going from pure hydrocarbon to pure acetonitrile is larger (724 cm-l energy increase) for this syst,em than for any other nitrile system. At least some of these occurrences seems explicable in terms of a more positive nature of the C atom of the nitrile group of acetonitrile compared with the similar atom of the other nitriles. A more positive C atom would result in a stronger interaction with the nonbonding electrons of the 0 atom of benzophenone. Thus, the energy of the n-electron orbital with the interaction (in acetonitrile) would be lowered more than in the case of interaction with any other nitrile. Therefore, further displacement of the n ---) ?r* band maximum would result in the case of acetonitrile (for additional causes and explanation, see the section on infrared). The lack of an isobestic point is indirectly explained by the same reason as that used to account for the greater energy change of t,he n + ?r* band maximum by acetonitrile than by other nitriles. Investigations by Lambert et al. (5) and Murray and Schneider (3) indicate that acetonitrile undergoes self association. The former authors actually measured the heat of dimerization of acetonitrile in the vapor state. Thus, in the case of acetonitrile there would not be a simple two species equilibrium with benzophenone in contrast to the situation with the other nitriles and benzophenone. This would be manifested by the lack of an isobestic point for the benzophenone-acetonitrile-tetrachloroethylene system.
12
BECKER
Instead, an isobestic region would result indicative of several species in equilibrium (benzophenone, benzophenone-acetonitrile monomer, benzophenoneacetonitrile dimer and/or polymer). Despite these differences, it is expected that the mode of interaction of acetonitrile with benzophenone would be the same as that of the other nitriles. Although no direct evidence for lack of self association of other nitriles exists, the results herein discussed and the results of the invest,igation of Murray and Schneider (3) lend strong evidence supporting this claim. 8. n-Propyl Ether. The lack of significant change in the shape of the benzophenone n + 8 band and the position of its maximum in varying percents of n-propyl ether (including 100 %) indicate little interaction between benzophenone and n-propyl ether. This is not an unexpected result in view of the nature of the n-propyl ether molecule. 4. Chloroform. The n + I? benzophenone band maximum undergoes a rather large change (767 cm-l increase) in energy in going from pure hydrocarbon to pure chloroform. In addition, there is indication of an isobestic point or region for the benzophenone-chloroform-hydrocarbon system. The form of the interaction is very likely to be in the nature of hydrogen bonding. This provides additional evidence regarding the protonic nature of the chloroform H atom to that previously noted, for instance by Murray and Schneider (3). The degree of the change of the n + ?r* maximum indicates a similar strength of interaction of chloroform and acetonitrile with benzophenone. Moreover, their interaction ability is in a position intermediate to the other nitriles studied and alcohol with chloroform lying somewhat closer to alcohol than does acetonitrile. In view of the different mode of interaction of chloroform and acetonitrile, it is not surprising that chloroform would have a slightly stronger interaction with benzophenone than acetonitrile. Infrared 1. Alcohol. The information gained in the infrared studies of benzophenonetetrachloroethylene-alcohol system has bearing on the interpretation of the other results and therefore will be considered first. The 1668.4 cm-’ carbonyl fundamental band undergoes an apparent shift to lower energy with increasing alcohol concentration. However, by the time the alcohol/benzophenone molar ratio is 25/l, a maximum shift of 1.7 cm-’ has occurred. Thereafter, any increase of the alcohol/benzophenone ratio has no effect in shifting the 1668 cm-l band even up to 100 % alcohol (85/l). However, the above situation is complicated by the appearance of a new band shoulder in the 1663-1656 cm-’ region at an alcohol/benzophenone molar ratio of 25/l (see Fig. 4). This band shoulder was not present at a molar ratio of 0.4/l. At higher alcohol/benzophenone molar ratios, this shoulder becomes a peak. In 100% alcohol, the peak is at 1656.3 cm-’ (see Fig. 5). During the in-
INTERACTION
OF POLAR SOLVENTS
13
crease in the alcohol-benzophenone ratio, the 1668 cm-l band decreases in intensity while the 1656 cm-* band increases in intensity (see Figs. 4 and 5). One explanation consistent with above facts is the following. First, it is assumed that the apparent shifting of the 1668 cm-l band is due to the presence of a new band building up a lower energy. The new band being assigned as due to a benzophenone monomer alcohol complex. Thus, at a low molar alcohol/benzophenone (hereafter referred to as a/b) ratio of 0.4/l because of an excess of benzophenone and a low association constant for the complex (Z), the concentration of the monomer complex is low. Therefore, practically no shift of the 1668cm-’ band is observed. At the next a/b ratio studied 25/l, several changes have occurred. The 1668.4 cm-l band has undergone an apparent shift to 1666.7 (change of 1.7 cm-l) and a new shoulder, missing at the low a/b ratio, has attained considerable intensity. Meanwhile, the former 1668 cm-l band has decreased in intensity. The band at 1666.7 cm+ could be assigned as a new band due to the monomer complex or as the original 1668 cm+ band apparently shifted because of the build up of a new band at lower energy due to monomer complex formation. The shoulder band at 1656 cm-l is assigned to a benzophenonealcohol dimer and/or benzophenone-alcohol polymer complex. Thus, the 1666.7cm-1 band is assigned to a benzophenone-complex monomer complex. The absence of any shoulder at a low alcohol concentration and low a/b ratio and the relative intensity changes of the bands would tend to substantiate the above conclusions. At a low alcohol concentration (0.08 M) and low a/b ratio (0.4/l) the complex formed would most likely be monomeric in nature and little self association of alcohol would take place. At a higher alcohol concentration (5.1 M) and a/b ratio (25/l), alcohol dimer and/or alcohol polymer type complexes would form, thus lowering the relative concentration of the benzophenone-alcohol monomer type as is reflected in the band intensity relationships. By the time the concentration of alcohol is even higher (100 % by volume or 17 M) where the a/b ratio is 85/l, the shoulder has become a band at 1656 cm-1 with further increase in int)ensity and accompanying intensity lowering of the 1666 cm-l benzophenone-alcohol monomer complex band. The situation is essentially the same for alcohol at a concentration of 95% where the a/b ratio is 54/l. These results would be in accord with the explanation in the preceding paragraphs. The difference between the 1668 cm-l carbonyl band (in tetrachloroethylene) and the 1656 cm-1 carbonyl band (in alcohol) is approximately 12 cm-l. This difference is similar to carbonyl frequency shifts obtained by other investigators (6’,7). A general literature search on the problem of the effect of alcoholic hydrogen bonding on the C=O group frequency indicated that the circumstance of two bands of nearly equal intensity in the C=O group frequency region did not
14
BECKER
occur. This situation could arise because of resolution problems and/or the IXture of the solvent-solute, and/or the solvent-solvent interactions. Thus, if no strong solvent-solvent interaction existed only one band would exist, that of the solute-solvent monomer complex. If on the other hand strong solvent-solvent interaction was possible, only one carbonyl band would exist, that of a solutesolvent polymer complex. If bands due to both solute-monomer solvent and solute-polymer solvent did exist, poor resolution would not indicate the true fact. Further consideration of the nature of the benzophenone-alcohol complexes that could be formed indicate the proper relative positions of the monomer and polymer assigned bonds. The complexing of one alcohol molecule to a benzophenone molecule would tend to lower the frequency of the carbonyl group band. This would result because of an inductive effect caused by the protonic aIcoholic hydrogen atom. The formation of alcohol dimer types complexes of the form \ /
C=O---H-0-CH2--CH3 : H-0-CHZ-CH,
tend to further increase the protonic nature of the keto bonded alcoholic hydrogen atom. Therefore, the carbonyl bonding electrons are subjected to a greater inductive pull than before causing a further change (decrease) of the carbonyl band frequency. The formation of higher order alcohol complexes should have only relatively minor added effects such as a band broadening effect. Consequently, in essence only two distinct bands would arise. In addition, the effect of a monomer solvation process on the carbonyl band frequency would appear to be less than that of a dimer type solvation process. Further, the dimer type solvation process would occur at only relatively high alcohol concentration. All of these facts are consistent with the intensity changes, intensity relations and relative positions of the bands found in this investigation. It is interesting to note that the explanation, as given above, indicates a fairly appreciable concentration of benzophenone-alcohol monomer complex exists even in pure alcohol. Finally, it should be noted that an alternative explanation could exist. This would assign the 1656 cm-’ band as due to monomer solvation. Thus, the 1666 cm-’ band would be the unperturbed carbonyl frequency (at 1668 cm-l) shifted by a general solvation effect.. The principal problems with this explanation are (a) considering the intensity of the 1666 cm-’ band, it seems unreasonable that uncomplexed benzophenone of the concentration indicated by this band could exist in 100% alcohol, (b) the fact that the 1668 cm-l band although shifting to 1666 cm-’ does not continue to shift upon further increase of the eth-
INTERACTION
OF POLAR
SOLVENTS
15
anol concentration as might be expected for a general solvation effect if it existed. In addition, it should be remembered that this investigation and that of Brealey and Kasha (2) indicated the apparent absence of solvation effects other than those of a specific nature as described herein. 9. Acetonitrile. In the case of acetonitrile, the 1668.4 cm-’ carbonyl band undergoes an apparent shift to lower energy with increasing acetonitrile concentration. By the time a nitrile/benzophenone molar ratio of 25/l is attained, the shift has reached its maximum value of 4.5 cm-l. Thereafter, there is no further shift of the band with increasing concentration up to a nitrilelbenzophenone ratio of 572/l. This shift is accompanied by a small decrease in intensity of the band as well as a small band broadening. The apparent shift of the band is not likely to be due to a general solvation effect since the shift ceases even though the concentration of acetonitrile is further increased by a factor of 3. The nitrile/benzophenone molar ratio is increased by a factor of 23 (see Table II). Although it might be that the shift would be predicted to cease because of the nature of the equations describing general solvation shifts, it does not seem plausible that the effect would be noticeable at the lower of the concentrations studies. Thus, the apparent shift is probably due to the fact that new bands have grown in at lower energy causing the apparent shift. The facts are in accord with the view that new species exist which have altered the carbonyl frequency. The nature of these species have been discussed in the ultraviolet section of Section IV. 3. Butyro- and Benzonitrile. The addition of butyro- or benzonitrile to benzophenone also causes an apparent shift of the 1668 cm-’ carbonyl band t’o lower energy. Here as in the cases of acetonitrile and alcohol, the shift reaches a maximum at a nitrile/benzophenone molar ratio of 25/l. The degree of the shift is 1.2 cm-‘. The cause of this shift would be the same as that for acetonitrile except that only a single new specie exists, that of a monomer nitrile-benzophenone complex. The fact that the shift is much less for these cases than for acetonitrile could be due to one or both of the following: (a) the nitrile-benzophenone complex formed is less stable than that formed by the acetonitrile, (b) since acetonitrile can dimerize, and additional dimer-type complex is formed whose band would appear at lower energy than the monomeric type. This would be similar to the case of alcohol where the dimer-type complex is shifted further to lower energy than is the monomer. Aeetonitrile can dimerize as indicated by other work (3,5) and the ultraviolet section of this investigation. The other nitriles seem incapable of dimerization as indicated by others (3) and the formation of a simple isobestic point, in the ultraviolet section of this work. The principal reason for the greater apparent change in the 1668 cm-’ band
BECKER
16
for the case of acetonitrile would appear to be due to the formation of an additional dimer-type complex of the acetonitrile-benzophenone. The dimer-type complex intrinsically signifies a complex of greater strength than the monomer type. This then would account for the apparent shift of the 1668 cm-’ band being (a) larger than for the other nitriles, (b) larger than for the alcohol-monomer type complex and (c) less than, but more nearly like that of the band due to t’he alcohol-dimer-type complex. Because of the difference in nature of interaction, it would be expected that dimer-alcohol-benzophenone complexes would produce greater inductive effects on the carbonyl group electrons than dimer acetonitrilebenzophenone complexes. This would result in a larger change of the carbonyl band frequency for the alcohol-type complex. Both the ultraviolet and infrared investigations indicate similar relative degrees of interaction of nitriles and alcohol with benzophenone. Thus, acetonitrile lies between the other nitriles and alcohol in affecting the change of the n -+ ?r* maximum to the blue. The greater change of the n + ?r* maximum caused by acetonitrile compared with other nitriles would appear in part to be due to the formation of an additional dimer-type acetonitrile-benzophenone complex. The justification of this conclusion would be based on arguments similar to those employed to explain the like phenomenon occurring for the carbonyl band. 4. E$ects on the Nitrile Group Frequency. The effect of the interaction of aceto-, butyro- and benzonitriles with benzophenone on the nitrile group band in the 2272 cm-1 region was examined. There was no notable shift of the nitrile group frequency upon interaction with benzophenone for any of these nitriles. In at least one instance, acetonitrile, studies were made up to a nitrile concentration of 14.3 M which corresponded to a nitrile/benzophenone molar ratio of 572/l. In this latter case, there was indication of a new band(s) at higher frequency, ACKNOWLEDGMENT
to Dr. Jose Fernandez and Dr. Eion McRae for helpful discussions regarding the results of this investigation. Also, to Mrs. Jean B. Allison for her help in obtaining some of the spectral data. The author
RECEIVED:
wishes to express
March
appreciation
31, 1958 REFERENCES
1. H. MCCONNELL, J. Chem. Phys. 20, 700 (1952). 2. G. J. BREALEY AND M. KASHA, J. Am. Chem. Sot. 77.4462 (1955). 8. F. E. MURRAY AND W. G. SCHNEIDER, Can. J. Chem. 33, 797 (1955). 4. E. G. MCRAE, J. Phys. Chem. 61, 562 (1957). 6. J. D. LAMBERT, G. A. ROBERTS, J. S. ROWLINSON, AND V. J. WILKINSON, Proc. Roy. Sot. A196, 113 (1949). 6. R. S. RASMUSSEN, D. D. TUNNICLIFF, AND R. R. BRATTAIN, J. Am. Chem. Sot. 71, 1068 (1949). 7. W. GORDY, J. Am. Chem. Sot. 60, 605 (1938).