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The vibrational spectra of propynoic acid and sodium propynoate
Spectrochimica
Acta, 1965, Vol.
21, pp. 1717to 1724. Pergamon PressLtd. Printedin
Northern
Ireland
The vibrational spectra of propynoic acid and sodium propynoate J. E. KATON Monsanto Research Corporation, Dayton 7, Ohio and NEIL T. MCDEVITT Air Force Materials Laboratory, Wright-Patterson
Air Force Base, Ohio
(Received 4 February 1965) Abstract-The vibrational spectra of propynoic acid and sodium propynoate are reported. These include both the infrared and Raman spectra in the region 4000-100 cm-l. Partial spectra of the deuterium-substituted compounds are also reported. The fundamental frequencies have been assigned in both compounds. INTRODUCTION ALTHOUGH & number
of workers have concerned themselves with the vibrational spectra of acetylenic compounds (see e.g. NYQUIST and POTTS [l] and references therein), very few detailed spectral studies have been made on acetylenic molecules containing a second functional group. To date these studies consist of propynal [2, 31 propargyl nitrile [4, 51, acetylene dinitrile [6] and propargyl alcohol [7]. Of these, only propynal possesses a nonlinear, conjugated system. Therefore, it is of interest to extend this series, and this paper reports the results of the study of the vibrational spectra of propynoic acid and sodium propynoate, two further examples of nonlinear acetylenic compounds in which the triple bond is conjugated with a carbonyl grouping. EXPERIMENTAL Materials
The propynoic acid was a commercial sample that was carefully redistilled under reduced pressure using a lo-in Vigreaux column. The propynoic acid-d, was obtained from Merck and Company of Canada and was also redistilled in the same manner. Sodium propynoate was prepared by the careful addition of a stoichiometric amount of aqueous sodium bicarbonate to propynoic acid. Water was removed by placing the sample in a vacuum desiccator for several days. Sodium propynoate-d, was prepared in a similar manner except that NaOD in Da0 replaced the aqueous sodium bicarbonate. [l] R. A. NYQUIST and W. J. Porrs, Spectrochim. Acta 16,419 (1960). [2] J.C.D. BRAND and J.K. G. WATSON,Trans. Famday Sot. 56, 1582 (1960). [3] G. W. KING and D. MOTJLE,Spectrochim. Acta 17, 286 (1961). [4] V. A. JOB and G. W. KING, Can. J. Chem. Qz, 3132 (1963). [5] G. C. TURRELL, W. D. JONES and A. MAKI, J. Chem. phya. 28, 1544 (1957). [6] F. A. MILLER and R. B. HANNAN, JR.,J. Chem. Phya. 21, 110 (1953). [7] P. TARTE and R. DEPONTEIERE, Bull. Sot. Chim. Belgea 06, 525 (1957).
2
1717
1718
J. E.
KATON and N.
T.
MCDEVITT
The vibrational spectra of propynoic
1719
acid and sodium propynoate
Spectra The Raman spectra typical spectrum of the salt in Fig. 2. The infrared spectra 301 and 421. A typical
38
,400
Fig.
I
3600
3400
Fig.
3000
A were obtained on a Cary Model 81 Raman spectrometer. acid is reproduced in Fig. 1 and of an aqueous solution of the were obtained on Perkin-Elmer spectrometers Models 102 [8], infrared spectrum of propynoic acid is reproduced in Fig. 3
2500
xx)c
1500
moo
500
3. The composite infrared spectrum of propynoic acid in the region 3600350 cm-’ (Ccl, and hexane solutions).
3clw
2500
2wo
mm
4. The infrared spectrum of sodium propynoate (KBr pellet).
1cw
500
in the region 3600-360
cm-l
and one of sodium propynoate in Fig. 4. The far infrared spectra of the two compounds are reproduced in Figs. 5 and 6. The infrared spectra of the two pure acids were so broad and featureless that it was found necessary to obtain them in fairly The dotted lines in dilute solutions (5 y0 by weight) in order to gain band definition. the spectrum represent solvent bands (Ccl,, hexane). The infrared spectra of the salts were run as Nujol mulls and alkali halide pellets. Infrared vapor spectra of propynoic acid were obtained in a ten cm cell heated to 5OOC. Propynoic acid decomposes on heating to acetylene and carbon dioxide, and this reaction appears to be catalyzed by metals. The most satisfactory vapor spectra were obtained with a glass-bodied cell. RESULTS The absorption bands of propynoic acid and sodium propynoate their proposed assignments are given in Tables 1 and 2. [8] N. T. MCDEVIYYCand W. L. BAUN, Spectrochint. Acta 20, 799 (1964).
together
with
J. E. KATON and N. T. MCDEVITT
1720
The assignments have been made by considering those of similar molecules [l-7], standard group frequencies [9], the deuterated molecules and the vapor band contours. The latter two techniques, although standard aids in the assignment of vibrational frequencies, are not as helpful in this case as usual. The hydrogen modes
0 350
300
250
200
150
100
50
WAVE NUMBER IN CM-’
Fig.
5. The far infrared spectrum
of propynoic
acid (capillary
film).
WAVE NUMBER IN CM-’
Fig.
6. The far infrared spectrum of sodium propynoate
(Nujol mull).
can be selected by comparison with other molecules with relative ease in most cases, so that deuteration does not provide much except confirmatory evidence. The facile decomposition of propynoic acid on heating prevents the recording of [9] L. J. BELLAMY The Infrared Spectra of Complex
Mobcules
(2nd Ed.) John Wiley
(1958).
The vibrational spectra of propynoic acid and sodium propynoate Table 1. The observed
vibrational absorption bands of propynoic propynoic acid-d,
HCsC-CO,H IR (solution)
3306 m
acid and
DCeC-CO,D
IR (vapor)
Type
3581 3568 I 3588 3329 3324
A
R (liquid)
IR (solution)
R (liquid)
Assignment
vOH 3297 m, b
3300-2700 m, vb max. at 3287, 3076, 2986, 2960, 2878 2636 w
vCH
vOH (polynw 2631 w, b 2600 m 2365-2215 m, vb max. et 2356, 2331, 2278, I 2218
2500 w
2130 m
1721
2137 2125 >
2610 B
vCD vOD
vcac
2129 “8, BP 2096 m 1987 8 1962 VW, sh 1870 m, b
2035 w 1980 8
VCESZ
1887 m
1796 w 1773 w 1740 w, sh 1731 WV,sh 1718 m, sh P 1676 8, b, P 1428 VW
1716 w, ah 1698 B
1700 8
1682 m
vc =o
1400 w 1401 YW
1400 m 1373 1362 1354 I
A
vco (monomer) vc-0
1327 8 1286 VW, sh
1300 TV,sh
1263 B
VG-0
1310 1300 1290 1
A
SOH (monomer) SON
1269 w, P 1262 m
1212 w, sh 1147 w-m
1231 VW, sh
1233 w 1181 w 1145 w-m
1162 1146
SOH (dimer)?
1072 w 1050 m
6OD
900 w 859 w
869 8, P 823 812 805
A
752 w-m 720 w
753 VW, sh 723 In 690
683 > 693
w
650 w-m
B
881 w 861 w
V’= 860 w
vc-c V’H (monomer) dOC0 6CH 6CH (monomer)
693 w ?‘CH
J. E. KATON
1722
and N. T. MCDEVITT
Table 1 (Co&d.) HCEC-CO,H IR (solution)
IR (vapor)
DCSC-CO,D R (liquid)
Type
IR (solution)
R (liquid)
Assignment
637 m 600 w
603 w
585 w
571 w, P
FD xc0 SCD
600 w 575 w
?JCD
563 VW 494 VW 487 w 470 VW 258* w 240* w-m
yoco
245 m 218 8
&CC yccc yH- - -OH
147* w * capillary film. 8 = strong, m = medium, w = weak, Y = very, 6 = in-plane bend, y = out-of-plane bend.
Table
b = broad,
P = polarized, sh = shoulder, Y = stretch
2. The observed vibrational absorption bands of sodium propynoate sodium propynoate-d, HCGG-C02Na
IR (KBr pellet) 3278 m
D-C-C-CO,Na R (H,O solution) 3292 w
IR (KBr pellet) 3267 m-m 2576 m
Assignment vCH vCD
2140 w 2095 w 2095 w-m
2102 vs, P 1957 m
1619 w, sh 1600 s
1626 m, b 1602 8, b 1570-1630 8, b
1490 VW 1471 YW 1382 s
1364 “8, P
891 m
900 8, P
789 8
798 w
io4 m
727 VW, sh 713 w 684 VW, sh
1366 s 1117 VW 1030 w
VkC VCEC a, YCO, a, vco,
“y, vco,
vc-c 893 888 789 770
TV,sh B s VW, sh
*c-c dOC0
6CH 662 m
652 m 602 w 582 w-m
606 In, P? 597 VW, sh
607 w 578 m 557 m
256” w-m 225* 206* 162* 122:
489 440 250 236
w, b w, b m VW, sh
508 m
w m-s w w
* Nujol mull. 8 = strong, m = medium, w = week, v = very, b = broad, P = polarized, symmetric, sy = symmetric, v = stretch, S = in-plane bend, y = out-of-plane bend.
YCH GCCO GCCO 6CD YCD yoco &CC
yccc lrtttice lett,ice sh = shoulder,
a = anti.
The vibrational
spectra of propynoic
acid and sodium propynoate
1723
a good vapor spectrum. At the temperatures available one cannot obtain a pure monomer spectrum, and it is possible that species higher than the dimer may also exist in the vapor. In all cases a broad, featureless band is noted in approximately the same position as a liquid phase band. In some cases a band possessing a definite contour appears, shifted somewhat from the broad band, and in other cases structure is noted that is obviously superimposed on a broad band. This makes the vapor spectrum very difficult to interpret. In general, it is noted that vibrations along the smallest principal axis (approximately the HC-C-C axis) yield bands that have only two branches (Type B). Other vibrations yield bands of varying nature, some of which appear to be similar to Type A bands and some similar to Type C. Because of band overlapping, however, it does not seem profitable to speculate on the exact nature of these bands. In Table 1 only the vapor bands with significant structure are listed. The Raman spectrum of propynoic acid-d, was of poor quality. The amount of material available was so small that only the 0.7 ml cell could be used, and fluorescence raised the background. The amount of sodium propynoate-d, available was insufficient for Raman studies. DISCUSSION The infrared spectrum of pure propynoic acid consists of several extremely broad bands with superimposed maxima. In fact, at ordinary thickness, the transmission seldom rises above 50% in the range 4000-500 cm-l. Only on diluting the material with a solvent is one able to gain sufficient band definition to attempt analysis of the spectrum. Even in fairly dilute solutions the bands tend to be broad with numerous shoulders. This behavior is indicative of an extreme case of hydrogen bonding and can best be explained by relatively strong hydrogen bonding of the acetylenic hydrogen to the oxygens of the carbonyl group. This, added to the normal carboxylic acid hydrogen bonding, results in the fact that only the two central carbon atoms, or two of the seven atoms, are not involved in a hydrogen bond system of the type K-H - - - Y. Even one of the central carbon atoms (the one bearing the triple bond) may be somewhat involved, since hydrogen bonding to pi electrons is a well-known phenomenon. This hydrogen bonding of acetylenic hydrogens has been previously studied by a number of investigators [2, 10-131. Consistent with this interpretation is the fact that the Raman spectrum of pure propynoic acid is somewhat less affected, and it is well known that hydrogen bonded modes are not so broad and strong in Raman spectra as in infrared spectra. Because of this excessive hydrogen bonding, the assignment is difficult and in many cases tenuous. Weak bands are particularly difficult since the existence of more than one hydrogen bonded species may give rise to different bands. These .could not be differentiated from overtones or combination bands. It does not, therefore, seem profitable to attempt to assign these weak bands. [IO] J. C. D. BRAND, G. EGLINTON and J. F. MORMON, J. Chem. Sot. 2526 (1960). [Ill A. A. PETROV and T. V. YAKOVLEVA, Opt. Spectry. (USSR) 7, 479 (1959). [12] A. A. PETROV, V. B. LEBEDEV and I. G. SAVICH, J. Qen Chem. (USSR) 32, 655 (1961). [13] A. A. PETROV, N. V. ELSAKOV and V. B. LEBEDEV, Opt. Spectry. (USSR) 16, 547 (1964).
J. E. KATON and N. T. MCDEVITT
1724
The assignment of the yCH vibration of propynoic acid and its monomer is consistent with the hydrogen bonding results of BRAND, EGLINTON and MORMON [lo], who found that hydrogen bonding raises the frequency of this vibration. The assignment of the skeletal frequencies has been made consistent with other data in the literature. The vC-C, 6CC0, WCC and yCCC vibrations are all similar to those of propynal (947, 620, 261 and 226 cm-l) [3] and the 6OCO and yOC0 are at about the frequencies expected from data on saturated acids [14]. The assignments of the first four skeletal frequencies are also consistent in the acid and the salt. The assignment of the 1147 cm-l band of propynoic acid to the dimer is somewhat speculative, but is consistent with experimental results. This band is considerably weaker than the 1263 cm-i band in solution, but the two are of roughly the same intensity (strong) in the vapor spectrum. It is difficult to explain both the frequency and intensity behavior in any other manner. Further evidence for this assignment was obtained from dilution studies. The results of these studies are tabulated in Table 3 and it is seen that the 1147 cm-l band increases in absorbance relative to the 1265 cm-l band with dilution. Table
3. Dilution study of the 1265 and 1147 cm-l
Cone. (vol. O/o in Ccl,) 2.5 1.25 0.625
bands of propynoic acid
Absorbance Path Length (mm)
1265 cm-l
1147 cm-l
0.05 0.10 0.20
0.83 0.92 0.84
0.04 0.05 0.06
126511147 21 18 14
It is of interest to note that the CEC and C=O stretching frequencies of propynoic acid and the C=C and both the antisymmetric and symmetric OCO stretching frequencies of sodium propynoate are within the normal range suggested for nonconjugated molecules [9]. This is expected in view of the large amount of previous work on chemical and physical properties of this type of compound which shows the lack of transmission of electrical effects by conjugated triple bonds (see e.g. KOCHI and HAMMOND [ 151, KATRITZKY and co-workers [ 16, 171 and FUCHS [IS]). Acknowledgement-We
wish to thank Messrs. FRED BEHNKE, W. ALLAN DAVIDSON for their aid in this work.
R. FEAIRHELLER, JR. and
[14] F. F. BENTLEY, M. T. RYAN and J. E. KATON, Spectrochim. Acta 20, 685 (1964). [15] J. K. KOCHI and G. S. HAMMOND, J. Am. Chm. Sot. 75, 3452 (1953). [16] A. R. KATRITZKY, D. J. SHORT and A. J. BOULTON, J. ClLem. Sot. 1516 (1960). [ 171 A. R. KATRITZKY, A. J. BOULTON and D. J. SHORT, Ibid. page 1519. [18] R. FUCHS, J. Org. Chem. 28, 3209 (1963).
Acta, 1965, Vol.
21, pp. 1717to 1724. Pergamon PressLtd. Printedin
Northern
Ireland
The vibrational spectra of propynoic acid and sodium propynoate J. E. KATON Monsanto Research Corporation, Dayton 7, Ohio and NEIL T. MCDEVITT Air Force Materials Laboratory, Wright-Patterson
Air Force Base, Ohio
(Received 4 February 1965) Abstract-The vibrational spectra of propynoic acid and sodium propynoate are reported. These include both the infrared and Raman spectra in the region 4000-100 cm-l. Partial spectra of the deuterium-substituted compounds are also reported. The fundamental frequencies have been assigned in both compounds. INTRODUCTION ALTHOUGH & number
of workers have concerned themselves with the vibrational spectra of acetylenic compounds (see e.g. NYQUIST and POTTS [l] and references therein), very few detailed spectral studies have been made on acetylenic molecules containing a second functional group. To date these studies consist of propynal [2, 31 propargyl nitrile [4, 51, acetylene dinitrile [6] and propargyl alcohol [7]. Of these, only propynal possesses a nonlinear, conjugated system. Therefore, it is of interest to extend this series, and this paper reports the results of the study of the vibrational spectra of propynoic acid and sodium propynoate, two further examples of nonlinear acetylenic compounds in which the triple bond is conjugated with a carbonyl grouping. EXPERIMENTAL Materials
The propynoic acid was a commercial sample that was carefully redistilled under reduced pressure using a lo-in Vigreaux column. The propynoic acid-d, was obtained from Merck and Company of Canada and was also redistilled in the same manner. Sodium propynoate was prepared by the careful addition of a stoichiometric amount of aqueous sodium bicarbonate to propynoic acid. Water was removed by placing the sample in a vacuum desiccator for several days. Sodium propynoate-d, was prepared in a similar manner except that NaOD in Da0 replaced the aqueous sodium bicarbonate. [l] R. A. NYQUIST and W. J. Porrs, Spectrochim. Acta 16,419 (1960). [2] J.C.D. BRAND and J.K. G. WATSON,Trans. Famday Sot. 56, 1582 (1960). [3] G. W. KING and D. MOTJLE,Spectrochim. Acta 17, 286 (1961). [4] V. A. JOB and G. W. KING, Can. J. Chem. Qz, 3132 (1963). [5] G. C. TURRELL, W. D. JONES and A. MAKI, J. Chem. phya. 28, 1544 (1957). [6] F. A. MILLER and R. B. HANNAN, JR.,J. Chem. Phya. 21, 110 (1953). [7] P. TARTE and R. DEPONTEIERE, Bull. Sot. Chim. Belgea 06, 525 (1957).
2
1717
1718
J. E.
KATON and N.
T.
MCDEVITT
The vibrational spectra of propynoic
1719
acid and sodium propynoate
Spectra The Raman spectra typical spectrum of the salt in Fig. 2. The infrared spectra 301 and 421. A typical
38
,400
Fig.
I
3600
3400
Fig.
3000
A were obtained on a Cary Model 81 Raman spectrometer. acid is reproduced in Fig. 1 and of an aqueous solution of the were obtained on Perkin-Elmer spectrometers Models 102 [8], infrared spectrum of propynoic acid is reproduced in Fig. 3
2500
xx)c
1500
moo
500
3. The composite infrared spectrum of propynoic acid in the region 3600350 cm-’ (Ccl, and hexane solutions).
3clw
2500
2wo
mm
4. The infrared spectrum of sodium propynoate (KBr pellet).
1cw
500
in the region 3600-360
cm-l
and one of sodium propynoate in Fig. 4. The far infrared spectra of the two compounds are reproduced in Figs. 5 and 6. The infrared spectra of the two pure acids were so broad and featureless that it was found necessary to obtain them in fairly The dotted lines in dilute solutions (5 y0 by weight) in order to gain band definition. the spectrum represent solvent bands (Ccl,, hexane). The infrared spectra of the salts were run as Nujol mulls and alkali halide pellets. Infrared vapor spectra of propynoic acid were obtained in a ten cm cell heated to 5OOC. Propynoic acid decomposes on heating to acetylene and carbon dioxide, and this reaction appears to be catalyzed by metals. The most satisfactory vapor spectra were obtained with a glass-bodied cell. RESULTS The absorption bands of propynoic acid and sodium propynoate their proposed assignments are given in Tables 1 and 2. [8] N. T. MCDEVIYYCand W. L. BAUN, Spectrochint. Acta 20, 799 (1964).
together
with
J. E. KATON and N. T. MCDEVITT
1720
The assignments have been made by considering those of similar molecules [l-7], standard group frequencies [9], the deuterated molecules and the vapor band contours. The latter two techniques, although standard aids in the assignment of vibrational frequencies, are not as helpful in this case as usual. The hydrogen modes
0 350
300
250
200
150
100
50
WAVE NUMBER IN CM-’
Fig.
5. The far infrared spectrum
of propynoic
acid (capillary
film).
WAVE NUMBER IN CM-’
Fig.
6. The far infrared spectrum of sodium propynoate
(Nujol mull).
can be selected by comparison with other molecules with relative ease in most cases, so that deuteration does not provide much except confirmatory evidence. The facile decomposition of propynoic acid on heating prevents the recording of [9] L. J. BELLAMY The Infrared Spectra of Complex
Mobcules
(2nd Ed.) John Wiley
(1958).
The vibrational spectra of propynoic acid and sodium propynoate Table 1. The observed
vibrational absorption bands of propynoic propynoic acid-d,
HCsC-CO,H IR (solution)
3306 m
acid and
DCeC-CO,D
IR (vapor)
Type
3581 3568 I 3588 3329 3324
A
R (liquid)
IR (solution)
R (liquid)
Assignment
vOH 3297 m, b
3300-2700 m, vb max. at 3287, 3076, 2986, 2960, 2878 2636 w
vCH
vOH (polynw 2631 w, b 2600 m 2365-2215 m, vb max. et 2356, 2331, 2278, I 2218
2500 w
2130 m
1721
2137 2125 >
2610 B
vCD vOD
vcac
2129 “8, BP 2096 m 1987 8 1962 VW, sh 1870 m, b
2035 w 1980 8
VCESZ
1887 m
1796 w 1773 w 1740 w, sh 1731 WV,sh 1718 m, sh P 1676 8, b, P 1428 VW
1716 w, ah 1698 B
1700 8
1682 m
vc =o
1400 w 1401 YW
1400 m 1373 1362 1354 I
A
vco (monomer) vc-0
1327 8 1286 VW, sh
1300 TV,sh
1263 B
VG-0
1310 1300 1290 1
A
SOH (monomer) SON
1269 w, P 1262 m
1212 w, sh 1147 w-m
1231 VW, sh
1233 w 1181 w 1145 w-m
1162 1146
SOH (dimer)?
1072 w 1050 m
6OD
900 w 859 w
869 8, P 823 812 805
A
752 w-m 720 w
753 VW, sh 723 In 690
683 > 693
w
650 w-m
B
881 w 861 w
V’= 860 w
vc-c V’H (monomer) dOC0 6CH 6CH (monomer)
693 w ?‘CH
J. E. KATON
1722
and N. T. MCDEVITT
Table 1 (Co&d.) HCEC-CO,H IR (solution)
IR (vapor)
DCSC-CO,D R (liquid)
Type
IR (solution)
R (liquid)
Assignment
637 m 600 w
603 w
585 w
571 w, P
FD xc0 SCD
600 w 575 w
?JCD
563 VW 494 VW 487 w 470 VW 258* w 240* w-m
yoco
245 m 218 8
&CC yccc yH- - -OH
147* w * capillary film. 8 = strong, m = medium, w = weak, Y = very, 6 = in-plane bend, y = out-of-plane bend.
Table
b = broad,
P = polarized, sh = shoulder, Y = stretch
2. The observed vibrational absorption bands of sodium propynoate sodium propynoate-d, HCGG-C02Na
IR (KBr pellet) 3278 m
D-C-C-CO,Na R (H,O solution) 3292 w
IR (KBr pellet) 3267 m-m 2576 m
Assignment vCH vCD
2140 w 2095 w 2095 w-m
2102 vs, P 1957 m
1619 w, sh 1600 s
1626 m, b 1602 8, b 1570-1630 8, b
1490 VW 1471 YW 1382 s
1364 “8, P
891 m
900 8, P
789 8
798 w
io4 m
727 VW, sh 713 w 684 VW, sh
1366 s 1117 VW 1030 w
VkC VCEC a, YCO, a, vco,
“y, vco,
vc-c 893 888 789 770
TV,sh B s VW, sh
*c-c dOC0
6CH 662 m
652 m 602 w 582 w-m
606 In, P? 597 VW, sh
607 w 578 m 557 m
256” w-m 225* 206* 162* 122:
489 440 250 236
w, b w, b m VW, sh
508 m
w m-s w w
* Nujol mull. 8 = strong, m = medium, w = week, v = very, b = broad, P = polarized, symmetric, sy = symmetric, v = stretch, S = in-plane bend, y = out-of-plane bend.
YCH GCCO GCCO 6CD YCD yoco &CC
yccc lrtttice lett,ice sh = shoulder,
a = anti.
The vibrational
spectra of propynoic
acid and sodium propynoate
1723
a good vapor spectrum. At the temperatures available one cannot obtain a pure monomer spectrum, and it is possible that species higher than the dimer may also exist in the vapor. In all cases a broad, featureless band is noted in approximately the same position as a liquid phase band. In some cases a band possessing a definite contour appears, shifted somewhat from the broad band, and in other cases structure is noted that is obviously superimposed on a broad band. This makes the vapor spectrum very difficult to interpret. In general, it is noted that vibrations along the smallest principal axis (approximately the HC-C-C axis) yield bands that have only two branches (Type B). Other vibrations yield bands of varying nature, some of which appear to be similar to Type A bands and some similar to Type C. Because of band overlapping, however, it does not seem profitable to speculate on the exact nature of these bands. In Table 1 only the vapor bands with significant structure are listed. The Raman spectrum of propynoic acid-d, was of poor quality. The amount of material available was so small that only the 0.7 ml cell could be used, and fluorescence raised the background. The amount of sodium propynoate-d, available was insufficient for Raman studies. DISCUSSION The infrared spectrum of pure propynoic acid consists of several extremely broad bands with superimposed maxima. In fact, at ordinary thickness, the transmission seldom rises above 50% in the range 4000-500 cm-l. Only on diluting the material with a solvent is one able to gain sufficient band definition to attempt analysis of the spectrum. Even in fairly dilute solutions the bands tend to be broad with numerous shoulders. This behavior is indicative of an extreme case of hydrogen bonding and can best be explained by relatively strong hydrogen bonding of the acetylenic hydrogen to the oxygens of the carbonyl group. This, added to the normal carboxylic acid hydrogen bonding, results in the fact that only the two central carbon atoms, or two of the seven atoms, are not involved in a hydrogen bond system of the type K-H - - - Y. Even one of the central carbon atoms (the one bearing the triple bond) may be somewhat involved, since hydrogen bonding to pi electrons is a well-known phenomenon. This hydrogen bonding of acetylenic hydrogens has been previously studied by a number of investigators [2, 10-131. Consistent with this interpretation is the fact that the Raman spectrum of pure propynoic acid is somewhat less affected, and it is well known that hydrogen bonded modes are not so broad and strong in Raman spectra as in infrared spectra. Because of this excessive hydrogen bonding, the assignment is difficult and in many cases tenuous. Weak bands are particularly difficult since the existence of more than one hydrogen bonded species may give rise to different bands. These .could not be differentiated from overtones or combination bands. It does not, therefore, seem profitable to attempt to assign these weak bands. [IO] J. C. D. BRAND, G. EGLINTON and J. F. MORMON, J. Chem. Sot. 2526 (1960). [Ill A. A. PETROV and T. V. YAKOVLEVA, Opt. Spectry. (USSR) 7, 479 (1959). [12] A. A. PETROV, V. B. LEBEDEV and I. G. SAVICH, J. Qen Chem. (USSR) 32, 655 (1961). [13] A. A. PETROV, N. V. ELSAKOV and V. B. LEBEDEV, Opt. Spectry. (USSR) 16, 547 (1964).
J. E. KATON and N. T. MCDEVITT
1724
The assignment of the yCH vibration of propynoic acid and its monomer is consistent with the hydrogen bonding results of BRAND, EGLINTON and MORMON [lo], who found that hydrogen bonding raises the frequency of this vibration. The assignment of the skeletal frequencies has been made consistent with other data in the literature. The vC-C, 6CC0, WCC and yCCC vibrations are all similar to those of propynal (947, 620, 261 and 226 cm-l) [3] and the 6OCO and yOC0 are at about the frequencies expected from data on saturated acids [14]. The assignments of the first four skeletal frequencies are also consistent in the acid and the salt. The assignment of the 1147 cm-l band of propynoic acid to the dimer is somewhat speculative, but is consistent with experimental results. This band is considerably weaker than the 1263 cm-i band in solution, but the two are of roughly the same intensity (strong) in the vapor spectrum. It is difficult to explain both the frequency and intensity behavior in any other manner. Further evidence for this assignment was obtained from dilution studies. The results of these studies are tabulated in Table 3 and it is seen that the 1147 cm-l band increases in absorbance relative to the 1265 cm-l band with dilution. Table
3. Dilution study of the 1265 and 1147 cm-l
Cone. (vol. O/o in Ccl,) 2.5 1.25 0.625
bands of propynoic acid
Absorbance Path Length (mm)
1265 cm-l
1147 cm-l
0.05 0.10 0.20
0.83 0.92 0.84
0.04 0.05 0.06
126511147 21 18 14
It is of interest to note that the CEC and C=O stretching frequencies of propynoic acid and the C=C and both the antisymmetric and symmetric OCO stretching frequencies of sodium propynoate are within the normal range suggested for nonconjugated molecules [9]. This is expected in view of the large amount of previous work on chemical and physical properties of this type of compound which shows the lack of transmission of electrical effects by conjugated triple bonds (see e.g. KOCHI and HAMMOND [ 151, KATRITZKY and co-workers [ 16, 171 and FUCHS [IS]). Acknowledgement-We
wish to thank Messrs. FRED BEHNKE, W. ALLAN DAVIDSON for their aid in this work.
R. FEAIRHELLER, JR. and
[14] F. F. BENTLEY, M. T. RYAN and J. E. KATON, Spectrochim. Acta 20, 685 (1964). [15] J. K. KOCHI and G. S. HAMMOND, J. Am. Chm. Sot. 75, 3452 (1953). [16] A. R. KATRITZKY, D. J. SHORT and A. J. BOULTON, J. ClLem. Sot. 1516 (1960). [ 171 A. R. KATRITZKY, A. J. BOULTON and D. J. SHORT, Ibid. page 1519. [18] R. FUCHS, J. Org. Chem. 28, 3209 (1963).