SOT., 10G1
hf E A S U R E Y E N T S
OF I N T R A M O L E C U L A R
OH group of energy near to that which exists in the liquid, (ac. obtained from shifts in OH stretching frequency in infrared) ; therefore the molar polarization of adsorbed methyl alcohol is near to that of thp liquid. Isobutane with its lorn dipole moment interacts much more strongly with the polar sur-
HYDROGES BONDIKG
2023
face OH groups than in the liquid state, and, consequently the adsorbate on these OH groups shows a higher polarization than the liquid. Acknowledgment.-The authors are indebted to Dr. M. E. Nordberg of the Corning Glass Works, Corning, N.Y., for the gift of the porous Vycor glass.
MEASURE11IENTS OF INTRAMOLECULAR HYDROGEN BONDING BY XUCLEAR MAGNETIC RESONBNCE AKD IXFRARED SPECTROSCOPY BY JOHNR. MERRILL Radiation Physics Laboratory, Engineering Department, E. I . du Pont de ,Vemozirs & Company, Wilmington, Delaware Received Mal/ 6 , 1961
The hydroxyl proton chemical shifts of a series of substituted 2-hydroxybenzophenones dissolved in carbon tetrachloride vary by 2.35 p.p.m. a t room temperature. The individual shifts change only about 0.3 p.p.m. from -25 to +80°, and are similarly insensitive to concentration. The infrared 0-H stretching bands have half-intensity widths of -400 cm.-l, and peaks at 2940 to 3200 cm.-l. A correlation is found between the hydroxyl shifts and the 0-H frequencies. Both hydroxyl groups of 2,2'-dihydroxybenzophenones appear to be chelated, although less strongly than those of the 2-hydroxybenzophenones.
Introduction Intramolecular hydrogen bonds between the carbonyl and hydroxyl groups of 2-hydroxyhenzophenones form six-atom chelate rings
measured with a precision of 3=0.5" and an accuracy of &lo.
All spectra were recorded from carbon tetrachloride solutions of the hydroxybenzophenones at concentrations of 0.3-1.0 $1. To avoid interactions with an added reference compound, hydroxyl shifts were calibrated with respect to O.--- HO other resonance peaks of each hydroxybenzophenone. All /I I major peaks were calibrated a t each temperature to establish that only the hydroxyl shiftschanged. The reference peaks were assigned their room-temperature shifts in C.P.S. a t 40 Mc. from external tetramethylsilane. The hydroxyl shifts from internal tetramethylsilane a t 23" also were determined, and found to be about 12 C.P.S. to higher field than those referred externally. Spectra were calibrated with audiofrequency modulation techniques. Three to six calibrations were made a t each The proton magnetic resonance and infrared spec- temperature and the results averaged. Tests of the calibratroscopy studies reported here concern the strengths tion procedures and the internal consistency of the shift of chelation in a series of these compounds. Com- data indicate that the measurements are accurate to a few petition between intramolecular and intermolec- tenths of a C.P.S. Resolution, expressed as the half-peak of a tetramethylsilane resonance, was about 1 C.P.S. ular hydrogen bonding is examined in a non- width Tnfrared absorption spectra were recorded on a Perkinhydrogen-bonding solvent. Chelation strengths Elmer model 221 grating spectrophotometer with a nominal are compared for derivatives with one and two resolution of 2 cm.-I. When possible, 0.3 M solutions in hydroxyl groups ortho to the carbonyl group and rarbon tetrachloride were used in cells of 0.1-mm. path length. For the less soluble derivatives, and for dilution with other substituents on the aromatic rings. studies, 1-10-mm. cells were used. S ectra also were taken The hydroxyl proton chemical shifts and their after deuterium had been substitutef for the hydroxyl hydependence on temperature and concentration1s2 drogen atoms. Exchange with deuterium was accomplished and the displacement of the 0-H fundamental by shaking a carbon tetrachloride solution of each hydroxywith a 200-fold molar excess of heavy water stretching frequencyZ are used to establish the benzophenone for a few minutes. The hydroxybenzophenones are relanature and relative strengths of the hydrogen tively insoluble in mater, as is carbon tetrachloride. After bonds. the two liquid phases were allowed to separate, the carbon tetrachloride solution of deuteroxybenzophenone was pipetExperimental Procedures trd off. The contribution of the C-H stretching absorptions Proton magnetic resonance spectra were obtained with a which overlap the hydrogen-bonded 0-H band can be seen Varian Associates V-4300B spectrometer a t a fixed frequency by comparing, or instrumentally subtracting, spectra of the of 40 Mc. A thermostat, similar in design and operation to normal and deuterated compounds (Fig. 1). Replacement that described by Rrownstein,3 mas constructed to fit the of the hydroxyl protons with deuterium was verified by Varian S M R probe for measurements a t controlled tempera- n.m.r. spectroscopy. tures. Samples r e r e spun in 5-mm. 0.d. glass tubes. The Results temperature near the sample was measured with a thermocouple located within the thermostat. A graph of differThe room-temperature hydroxyl proton shifts ences between this temperature and that of a liquid s a m p l ~ (Table I) range from 409 to 503 c.p.s. toward low was constructed for correcting the thermostat readings taken during calibration of spectra. Temperatures were field from internal tetramethylsilane, or from -0.23 (1) J. A. P o d e , I\. G . Schneider a n d H. J. Bernstem, "Hlghresolution Nuclear 3Iannetic Resonance " LIcGraa-Hi11 Book Co.. I n c , New York, N. 1 ' , 1959 p 400 ( 2 ) G . C. Pimentel and A L. JlcClellan, "The Hydrogen Bond," W. H. Freeman and Po., San Francisco, Californla, 1960. ( 3 ) S. Bronnetein, Can J . Chem., 37, 1119 (1959).
to -2.58 p.p.m. on Tiers' scale.4 All the 2hydroxyl peaks were narrow, with half-peak widths of 2 c.p.s. except that of 2,2'-dihydroxybenzophenone, which was 5 c.p.s. in width. The hy(4) G . V. D. Tiers, J . P h y s . Chem., 68, 1151 (1958)
JOHK R. MERRILL
2024
Vol. 6.5
TABLE I INFR4RED
0-H
OF
Comiiound
0-n STRETCHING FREQUEYCIES
.4ND HYDROXYL PROTON RESONANCE SHIFTS HYDROXYBENZOPHEXONES DISSOLVED IN CARBON TETRACHLORIDE
AND
-benzophenone
2-OH,O em. -1
Z-OH/Z-OD (ratio)
4-OH;b cm.-
4-OH/4-OD (ratio)
2-OH,C 0.P.S.
16
2,2',4,4'-tetrahydroxy-5,5'-di3200 1.33 3595 1.36 d t-butyl17 2,2 '-dihydroxy3200 1.33 ... ... 415 11 2,2',4-trihydroxy3190 1.34 3598 1.36 d 3190 1.35 . . ... 459, 409 7S, B 2,2'-dihydroxy-4-methoxyd 14 2,2 '-dihydroxy4,4'-dirnethoxy3150 1.34 ... ... 3100 1.34 ... 471 5 2-hydroxy-5-chloro13 2,2 '-dihydroxy--4,4'-didodecyloxy3100 1.32 ... 449 10 2,4-dihydroxy-5-hexyl3010 1.34 3595e 1.36 50 1 2960 1.33 . . ... 502 4 2-hydroxy-4-methoxy2940 1.32 .., .. 503 8 2-hydroxy-4decyloxyBest value for center of broad infrared absorption band (half-intensity widths cu. 400 cm.-l). b Non-hydrogen-bonded Too insoluble in CC14 &hydroxyl groups. c Down-field n.m.r. shift from internal tetramethylsiiane a t 40 Mc. and 23". for measurement. e At high conrentrations, intermolecularly hydrogen-bonded 4-hydroxyl groups produce a peak centered a t 3325 cm.?. I
.
.
Q
frared spectra of 2-hydroxybensophenones, even a t low concentrations. The chelated hydroxyl groups 041 have 0-H stretching bands displaced to lower frew 031 quencies, with half-peak widths of about 400 cm.-1. The corresponding 0-D bands are about 300 cm.-' in width. The broadness of the bands, the imperfect subtraction of interfering C-H peaks, and the partial overlap of 0-H and 0-D absorptions combine to limit the accuracy with which the 2000 2500 3000 3500 bands can be located. The best estimates of bandFREQUENCY c m d center frequencies for the derivatives studied range Fig. 1.-Infrared spectra of normal () and deuter2,2'-dihydroxy-4-methoxybenzophenone in from 2940 to 3200 cm.-I (Table I). The index ated (---) CCla. Concentration, 0.3 M ; cell path, 0 1 mm. numbers in Table I were assigned to the derivatives arbitrarily, to identify points on the graphs. droxyl proton chemical shifts are plotted against The maximum uncertainty in locating the center temperature in Fig. 2. For all compounds, a de- of each 0-H band is indicated by the horizontal crease in temperature causes the hydroxyl shifts to lines in Fig. 3. In most cases the centers probably move to lower field, the direction associated with have been located more closely. The 0-H/O-D stronger or more complete hydrogen bonding. frequency ratio is normally reported within the The changes are small, approximately 10 c.p.s. range of 1.29-1.3S.5 This ratio was calculated for over the available temperature range of -25 to each hydroxybenzophenone after both the 0-H +SO". and 0-D peak locations had been determined. It The hydroxyl shifts are also relatively insensitive varies from 1.32 to 1.35 for 2-hydroxyl groups, and to concentration. For example, the shift of 2- is near 1.36 for 4-hydroxyl groups (Table I). The hydroxy-4-decyloxybenzophenone undergoes an S- relative constancy of this ratio suggests that the C.P.S. decreaw upon eighteen-fold dilution. Both peaks have been located accurately. &4tan 0-H hydroxyl peaks of 2,2'-dihydroxy-4-methoxybenzo- frequency of 3000 cm.-I, a ratio change from 1.32 phenone, which is less soluble, behave similarly to 1.35 corresponds to a frequency change of 70 (Table 11). The direction of the changes indicates cm. -I. less hydrogen bonding at lower concentrations. To investigate the possible confusion of 2-hyKO free hydroxyl peaks were observed in the in- droxyl bands with hydrogen-bonded 4-hydroxyl bands in some of the derivatives, spectra were TABLE I1 taken from concentrated and dilute solutions of INFLUEUCE OF COXCENTRATIOU ou HYDROXTL SHIFT' 2,4-dihydroxy-5-hexylbenzophenone. At 0.5 ill, a 2-Hydi oxy-4-derylouyband attributable to intermolecular hydrogen benzophenone 2 2'-Dihydroxy-4-methoxybenlophenone Hx droxvl bonding is prominent a t 3325 cm.-l. At 0.01 M , Concn.. shift Concn.. Hydrovyl shift,c this band has virtually disappeared, and most of 31 C P S JI C P S the 4-hydroxyl groups contribute to a narrow free 3 5 518 1 5 474 424 peak at 3595 cm.-I. The other 4-hydroxyl com2.3 516 1 0 47 1 421 pounds examined have maximum solubilities near 1 2 514 0 5 470 420 0.01 X. Interference from a bonded 4-hydroxyl 0 6 512 0 3 470 420 hand does not appear to be a major problem a t 0 2 510 these low concentrations. Down-field Measured in carbon tetrachloride a t 21". shift from external tetramethylsilane (calibrated from niajor X o fine structure could be definitely ascribed to -CIOH~Ipeak, assigned a constant shift of 68 r.p s ). e the 0-H or 0-D absorptions, which are single, 067 05t
4
Down-fidd shift from external tetramethylsilane (calibrated from -OCH, peak, assigned a constant shift of 168 c.P.s.).
( 5 ) Reference 2, p. 112.
Sov., 1961
MEASUREMENTS OF INTRAMOLECULAR
fairly symmetrical bands. Spectra of normal and 2,2'-dihydroxy-4-methoxybenzophedeuterated none are shown in Fig. 1. The carbonyl group C=O stretching absorption of benzophenone is near 1670 cm.-l. Broader, structured C=O bands from the hydroxybenzophenones occur a t 1620-1640 cm.-l. The direction of this frequency shift is that expected for hydrogen bonding.2 The carbonyl peaks are not sufficiently consistent in shape to be located with the precision required for comparison with the hydroxyl-group infrared or n.m.r. shifts. These frequencies are similar to those reported for crystalline anthraquinone and its hydroxy derivatives.6 Broad 0-H stretching bands of some chelated hydroxyquinones have been assigned near 2900 cm.-l.'
HYDROGEN BONDING
2025
w
Discussion The chemical shift of the hydroxyl proton of a 2-hydroxybenzophenone can be expected to arise in a complex manner. Diamagnetic currents induced in the adjacent aromatic r i r ~ g , and ~ , ~ the angle, strength and extent of formation of hydrogen bonds are factors that should influence the observed shifts. If the subsidiary contributions are similar for all the derivatives, it is possible that the hydroxyl shifts are related monotonically to the hydrogen-bond strengths. As shown in Fig. 2, the individual 2-hydroxyl shifts are all relatively insensitive to temperature, changing only some 10 C.P.S. over a range of about 100'. This behavior contrasts with that of an intermolecularly hydrogen-bonded 4-hydroxyl group, whose shift changes about 1 c.p.s. per degree at the concentrations employed here. Similarly, the 2-hydroxyl shifts are relatively independent of concentration, as shown in Table 11. Although the 2-hydroxyl shifts include a small intermolecular contribution at high concentrations, they primarily reflect the strength of chelation. The infrared spectra also support virtually complete chelation of the 2-hydroxyl groups. Their 0-H stretching absorptions are displaced some 400 to 660 cm.-' from the free 0-H frequency near 3600 cm.-'. and are broadened some twenty-fold. Even a t concentrations of 0.01 M , no free 0-H peak is detected for the 2-hydroxy compounds. In the unsymmetrically substituted compound 7 (2,2'-dihydroxy-4-methoxybenzophenone)residence times for the two hydroxyl proton sites are sufficiently long to give two narrow resonance peaks separated by 50 c.p.s. Despite this difference in shift, the behavior of both peaks meets the criteria for chelation. Furthermore, the infrared 0-H and 0-D bands each appear single and no broader than those of other derivatives (Fig. 1). No changes are found in infrared spectra taken a t concentrations from 1.5 to 0.01 M that would suggest intermolecular hydrogen bonding of one of the hydroxyl groups. Although the separation of the 1i.m.r. peaks may indicate different chelation (6) M. St. C. Flett, J. Chem. SOC.,1441 (1948). (7) D. Hadzi and N. Sheppard, Trans. Faraday Soc., 60, 911 (19B4) (8) Reference 1, p. 181. (9) L. UT.Reeves, Can. J. Chem., 38, 748 (1960).
415 -40
I
I
-20
t
I
I
I
I
0 +20 +40 TEMPERATURE, "G.
I
I
+60
I
-1i +6@
Fig. 2.-Hydroxyl proton chemical shifts of 2-hydroxybenzophenes us. temperature. The derivatives are identified by number in Table I. 5201-
$ 500In
i? 480-
.
r6
>
45
460-
440-
-1 > X
p 9
420-
400 3300
3200
3100 3000 2900 0-H STRETCHING FREQUENCY, Em-'.
Fig. 3.-Correlation of hydroxyl proton chemical shift with infrared 0-H stretching frequency. Compounds are identified in Table 1.
strengths for the two ortho-hydroxyl groups, other magnetic shielding effects also may contribute. The 0-H infrared absorption bands cannot be located with the same accuracy as the n.m.r. resonances. However, the symmetry of the 0-H bands, and the previously mentioned isotopic shift between their frequencies and those of the narrower, more readily located 0-D bands are helpful in establishing band-center frequencies. The lines in Fig. 3 indicate the maximum uncertainty in locating the centers through inspection of each
2026
Vol. 6.5
WILLIAMA. BARBER AND CAROL L. SLOA?;
0-H band itself. (C-H interference varies with the different compounds). The actual centers are more likely to occur near the middle of each range. Figure 3 shows that the larger chemical shifts correspond to lower 0-H frequencies. Each of these respective trends is associated with stronger hydrogen bonding. A similar correlation has been found by Reeves, Allan and Strgimme for intramolecular bonds in phenols and naphthols. lo Figure 3 and the additional infrared data in Table I indicate that the 2-hydroxybenzophenones have the strongest hydrogen bonds. Double chelation of the carbonyl group in 2,2'-dihydroxybenzophenones appears to reduce the strength of each bond, although not markedly. Again, the relative independence of the n.m.r. and infrared parameters to temperature and concentration indicates that both hydroxyl groups in these compounds are chelated. According to Stuart-Briegleb atom models, coplanarity of the two aromatic rings with the carbonyl group of benzophenone is prevented by steric hindrance between the 6- and 6'-hydrogen atoms. The minimum angle between rings appears to be about 30'. The hydroxyl group and its phenyl ring, and the carbonyl group of 2-hydroxy(10) L. W. Reeves, E. A. Allan and K. 0. Strgmme, Can. J. Chem., 38, 1249 (1960).
benzophenone can be coplanar. In a 2,2'-dihydroxybenzophenone, simultaneous planarity of both rings and their hydroxyl groups with the carbonyl group is impossible without distortion of bond angles. To line up with the carbonyl oxygen atom, each hydroxyl group must turn out of the plane of its phenyl ring. A 15O-offset of each ring from the carbonyl group would seem to be the required compromise, a t least for those derivatives in which both rings are identically substituted. It was noted that the separate hydroxyl proton resonances from compound 7 may arise because of unequal hydrogen-bond strengths. In this and other unsymmetrically substituted 2,2'-dihydroxybenzophenones, the phenyl ring whose hydroxyl group is more strongly chelated may be held more nearly coplanar with the carbonyl group than the other ring. A steric hindrance to planarity could partly account for the weaker chelation of each hydroxyl group in the 2,2'-dihydroxybenzophenones. Acknowledgments.-Helpful discussions were held with W. D. Phillips of our Central Research Department, and with H. Kobsa of the Pioneering Research Laboratory, Textile Fibers Department and R. Dessauer of Jackson Laboratory, Organic Chemicals Department, who also supplied the compounds for this study.
SOLUBILITY OF CALCIUM CARBIDE I N FUSED SALT SYSTEMS BY WILLIAMA. BARBERAND CAROLL. SLOAN Central Research Division, American Cyanmid Co., Stamford, Connecticut Received M a y 1.9, 1981
Calcium carbide, CaC2, has been found to be soluble in a number of pure alkali and alkaline earth salts and salt mixtures a t temperatures up to 1000'. The highest solubilities were observed with lithium salts. The variation of solubility with temperature has been determined in several of these solvents. Measured solubilities are compared with those predicted theoretically for the ideal case.
Introduction Informatmionon the solubility of CaC2 is exceedingly scarce. No material which is liquid a t or near room temperature is known to dissolve CaC2. It has been mentionedl incidental to some work on Li2C2 that CaCz is soluble in some hydride-containing melts, but no quantitative information has been presented. It is known that CaCz forms eutectics with Ca02and with CaCXz,3but, because these mixtures melt above 1000°, they have limited utility. We now wish to report the results of a study concerning the solubility of calcium carbide in fused alkali and alkaline earth salts and their mixtures below 1OOO". Experimental Apparatus.-For pre aring solutions a vertical, electrically heated furnace (Levi-Duty Electric Co.) was used which accommodates a cylindrical tube of 1.25 inches 0.d. Since Pyrex or Vycor glass containers were found to be attacked by some melts, especially those containing Li+, the experiments were performed using stainless steel tubes. Although stainless steel is not completely inert to the melt, (1) A. Guntz and F. Benoit, Compt. rend., 176, 970 (1923). (2) G. Fluein and C. Aall, ibid.. 801, 451 (3) H. Franok and H. Heimann, 2. EZelFtrochsm., 88, 469 (1927).
(1935).
no significant amounts of corrosion products appeared in the filtered mixtures (determined by ultraviolet emission spectroscopic analysis: Fe < 1 p.p.m.; Cr, Ni, Mn not detectable). The apparatus used is a modification of that deecribed by Solomons, et u E . , ~ and is pictured in Fig. 1 . The melt was supported either on a coarse porosity Micrometallic stainlesa steel filter or a solid stainless steel disc perforated with 1 / 3 2 1 1 holes. Since the remainder of the apparatus was of Pyrex glass, the joints of the furnace tube were wrapped with water-cooled copper coils to protect the glass connections from thermally caused stress. Temperatures were measured with a Chromel-Alumel thermocouple inserted in a stainless steel thermocouple well and read on a Leeds & Northrup Type K potentiometer. Materials.-Argon gas (Linde High Purity grade) was further dried bv Dassine it through activated alumina (medried a t 400' for keverar hours). The calcium carbide used was ordinary commercial grade material (about 85% pure). The major impurity, CaO, was found to be insoluble in most of the melts studied. The carbide was broken into approximately 1/s to 1/4 inch lumps a t the time of use, and finer material was discarded. Reagent grade salts were used without further urification. LiCl and CaCL were placed in graphite crucibfes and premelted in a Lindberg crucible furnace under a flow of argon gas. This procedure served to remove any water picked up in handling and provided blocks of salt which were easier to keep dry. Any additional water present in the salts was
-
(4) C. Sglomons, et ol., J. Phys. Chew., 62, 248 (1958).
~.
