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The conformation and dipole moment of propiolic acid deduced from its microwave spectrum
Acts,Vol.28A,pp.1423to 1427.Persamon Prslls1972.Printed inNorthern Ireland
&e&o&imica
The conformation and dipole moment of propiolic acid deduced from its microwave spectrum D. G. LISTER end J. K. TYLER Chemistry Department, The University, Glwgow W.2 (Received3 Atqua! 1971) A~Miorowave spectra have been observed for propiolic acid and its two monodeutero species. The moleoule is found to be planar with the hydroxyl proton adopting the expected cis geometry with respect to the oarbony group. A structure consistent with the observed moments of in&k is proposed. The dipole moment hm been mm d ss l-69 A, 0*03D snd is almost parallel to the oarbonyl bond.
acid may be considered to be derived from formic acid by replacing the formyl proton with an acetylene group. One might expect conjugative effects between the carbonyl and acetylenic functions to be revealed in the molecular structure and in the msgnitude and direction of the dipole moment. Further, there exists the formal possibility of internal rotation of the hydroxyl group although it would be anticipated thst the crtrboxylic proton would adopt the planar, cis geometry with respect to the carbonyl group as in formic acid. To investigate this situation we have observed the microwave spectra of propiolic acid and its two monodeuteriated species.
PROPIOLIC
EXPERIMENTAL
All spectra were observed at room temperatures using a conventional 100 kHz. Stark modulation spectrometer. Laboratory grade propiolic acid supplied by KochLight Laboratories, Ltd. was distilled under reduced pressure and used without further purification. HCCCOOD was prepared in the wave guide cell by mixing propiolic acid and D,O vapours. Recordings of regions where HCCCOOD lines were predicted to occur were made with normal propiolic acid and then with the acid/D,0 mixture in the cell. The HCCCOOD lines were identified by comparing the two sets of recordings. DCCCOOH was prepared by the following method. A 50 % aqueous solution of propiolic acid was neutralized with dilute sodium hydroxide solution st 0°C. The water was removed under reduced pressure at room tempersture and the residual sodium propiolate (1 g) dissolved in D,O (3 ml). The pH of the solution wss sdjusted to 12 by adding a few drops of a solution of sodium deuteroxide in D,O and the mixture allowed to stand for two days et room temperature. After removing the water under reduced pressure the residue was dissolved in Hz0 (3 ml). The solution was cooled to O”C, a few drops of concentrated hydrochloric acid added and the propiolic acid extracted into ether. The ethereal solution was dried with anhydrous magnesium sulphate, filtered and the ether removed under reduced pressure. 1423
D. G. LISTER and J. K. TYLER
1424
ANALYSIS
OF SPECTRA
the purposes of predicting spectra a model for propiolic acid was set up based on the accurate data available for propynal [l] and formic acid [2]. This suggested that propiolic acid is a near prolate asymmetric rotor with KN -03, having an appreciable ,u~component of the dipole moment and a smaller ,u,,component. The strongest, readily accessible transitions were predicted to be the ,u,,, J,,+, f- J,,J and Jz,J_a c J1,J_l, Q-branch series together with the p,,, R-branch and a few pa, R-branch lines. Initial assignments followed the usual procedure of resolving Stark effects and making Q- and R-branch plots. Attempts to fit line frequencies to rigid rotor theory revealed the need to allow for a small amount of centrifugal distortion in the higher J, Q-branch lines. A least-squares fitting technique to (A + C)/2, (A - C)/2, K Tag Tbbbb* r~~cc and *dab was adopted using expressions for the centrifugal distortion energy in the form given by SORENSEN [3]. The expectation values of the quartic angular momenta terms were calculated from the formulae of WATSON [4]. A comparison of the observed and calculated line frequencies is given in Tables l-3 and the Par
Table 1. Line frequencies for HCCCOOH
Oberved 111+ OcKl %s+
101
211+
11,
41s +
31,
4,s +
38,
308 + 20, 31s+ 20, %1+ 21% 308 + 21, 31,+- 31, %o+- 21, 31,+ 211 3m+ %I 3II + %o 404 i-. 30, 401- 31, 3,1+- 31, 428 +
3,l
48, +
331
4a1+
3*0
41s +
404
4,s +
41,
514 +
50.5
5,s +
514
324 +
616
%s + 8 96 +
71,
%7 +
918
81,
16194.60 21363.44 16626.22 21302.43 27041.23 27076.23 14302.49 20041.03 23988.46 23222.36 21693.73 22086.47 27979.80 22241.36 22851.45 30832.76 28847.66 29792.68 29106.63 29141.26 14969.83 21810*89 18886*81 21199.49 21320.67 22416.27 24667.72 28196.47
(MHz)
Rigid rotor
Centrifugal distortion ensrgy
Celwleted
16194.69 21303.67 16626.32 21302.47 27041.28 27076.78 14302.40 20041.20 23988.81 23222.70 21694.33 22086.18 27980.32 22241.61 22862.29 30833.42 28848.31 29793.47 29108.17 29143.01 14960.74 21812.34 18888*10 21201.48 21323.56 22419.86 24672.32 28201.73
-0.04 -0.11 -0.16 -0.08 -0.22 -0.66 0.00 -0.14 -0.64 -0.31 -0.68 -0.64 -0.18 -0.05 -0.87 -0.64 -0.86 -0.99 - 1.73 - I.74 -0.79 - I.33 -1.29> - 1.92 -2.66 -3.69 -4.G -6.20
16194.66 21363.47 16626.16 21302.40 27041.06 27076.12 14302.40 20041.07 23988.27 23222.40 21693.76 22086.64 27980.14 22241.47 22851.42 30932.88 28847.46 29792.48 29106.44 29141.27 14959.96 21811.01 18886.81 21199.67 21320.90 22416.26 24667.67 28196.63
C. C. COSTAIN ELnd J. R. MORTON, J. Chem. Phys. 31, 389 (1959). [2] G. H. KWEI EtndR. F. CURL, J. Chem. phys. 35,1592 (1960). [3] G. 0. SORENSEN, J. Mol. Spectry 22, 325 (1967). [4] J. K. G. WATSON,J. Chem. Phy8. 46, 1935 (1967). [I]
Microwave spectrum of propiolic acid
1425
Table 2. Line frequencies for HCCCOOD (MHz)
Observed 214+
101
3,s + 2,1+
20, 21,
3111+ 21, 2ao'211 31,+211 4Id-+ 30.9 4Ol+- 31, 414- 318 3!u+ 31, 4s*+-381 481+- 330 4as+ 418 6!d11+614 6IS+--%, %4+ %4 7 14 -707 'SK +
'14
‘%e+%, %,-914
20546*10 20666.15 26655.00 19447.59 23621.02 22602.56 31630.83 21469.46 26533.65 22619.44 25216.40 25247.20 21497.66 20871.06 23099.66 20931.01 25566*6’ 2191oJ79 23957.25 2’251*69
Rigid rotor
20546.10 20666~10 26655.26 19447~50 23621.11 22602.56 31630.54 21649.10 26533.43 22619.76 25216.35 25245.25 21495.36 205'2.41 23100.96 20933.14 25565.49 21913.75 23991.26 2’256.63
Centrifugal distortion energy
+0*01 $-OS13 -0.20 +0*05 -0.0' -0.09 +0*14 +0*37 +0*22 -0.3' -1.05 -1.09 -0.50 -1.35 -1.43 -2.09 -1.50 -2.93 -3.55 -4.99
Calculated
20546.04 20666~23 26658*06 19447.5' 23621.04 22602.7' 31630.95 21469.63 26533.64 22619.39 25216.30 25247.30 21497.66 205'1.03 23099.63 20931.06 2556&69 21910.56 23957.37 2’25143
derived constants are given in Table 4. The rotational constants are well determined from the transitions studied, but little reliance can be placed on the centrifugal distortion constants (e.g. raabaand rcccc for HCCCOOD have both apparently positive values). This situation is largely due to the small centrifugal corrections involved in these particular transitions. A number of weaker vibrational satellites are observed to accompany each ground state transition, but no accurate measurements have been made. No evidence for other than a single rotameric form of propiolic acid has been found in this work. Table 3. Line frequencies for DCCCOOH (MHz)
Observed 21, +
303 313
101
+-202 +202
322 +221
321 +220 404 + 303 4 04 +
313
3 21 -
312
413 +
312
4 32 -
331
523 - %4 6 24 +%s '25 +
'16
826 +
817
92, +
913
20808.69
19871.11 26176.60 20167.31 20443.84 26178.12 19873.04 23896.40 28660.70 27010.11 22076.74 21821.64 2230557 23699.09 26131.11
Centrifugal distortion energy
Calculated
20808.80 19871.29 26176.82 20158~00 20444.70
-0*13 -0.19 -0.33 -0.69 -0.72
20808.67 19871*10 27176.49 20157.31 20443.98
19873*21 23896.85 28561.44 27012.02 22078.22 21823.70 22308.43 23702.77 26136.94
-0.30 -0.46 -0.74 -1.96 -1.45 -2.01 -2.86 -3.75 -4.80
19872.91 23896.39 28560.69 27010.06 22076.78 21821.60 22306.57 23699.02 26131.13
Rigid rotor
1426
D. G. LISW
and J. K. TYLEB
Table 4. Rotstionsl oonstents and centrifugal distortion constants (MHz)
A B
0 rluzaa ?bbb
Tcccc 7abab
HCCCOOH
HCCCOOD
DCCCOOH
12110~09 4146.94 3034.49 -0.1476 -0.0076 -0*0025 -0.0426
11858.32 401569 2995.58 -0.0172 $0~0013 +0.002s -0.0563
12109.93 3819.71 289963 -0.0355 -0~0101 -0*0060 -0.0160
CONFORMATION AND STRUCTURE
The moments of inertia and the inertial defects of the three isotopic species of propiolic acid are given in Table 5. The positive values for the inertial defects indicate that the molecule is essentially planar. The substitution co-ordin&es of the hydrogen atoms in the principal rtxis system of HCCCOOH were calculated from the Ib and I, values and are comprtred in Table 6 with these parameters calculsted from the structure given in Fig. 1. This structure is based on the accur&ely known data for propynal [l] and formic acid [2] and reproduces the observed moments of inertia quite closely. There is no doubt that the hydroxyl proton adopts the cis configuration with respect to the carbonyl group and we believe that the geometry shown in Fig. 1 is a reasonable approximation to the structure of propiolic acid in the gas phase. DIPOLE MOMENT
Stark effect measurements were made on the l,, c O,, and 3,, t 3,, transitions of HCCCOOH and the 2,, t l,, and 3,, c 3,s transitions of HCCCOOD. The 2 + 1 transition of OCS was used for calibration taking the dipole moment of this molecule as 0.7124D. All Stark displacements studied in propiolic acid were found to be proportional to E2 and the usual second order perturbation treatment [5] is adequate in this case. For both HCCCOOH and HCCCOOD ,u,,and ,ubwere obtained by graphically solving the appropriate simultaneous equations and the results are summarized Table 5. Moments of inertia snd hydrogen atom aoordinates
43 HCCCOOH* HCCCOOD* DCCCOOH* Model structure
Ib
41.7447 42.6308 41.7452 42.1126
121.9052 152*8897 132.3484 120.9109
Hydroxyl H Observed Model structure
IO 1636929 168.7590 174.3432 163.0235
A (a.m.u. A) 0.2430 0.2369 0.2496 -
Acetylenic H
o
b
a
b (A)
1.993 1.963
-0.968 -1.022
-3.244 -3.232
-0.024 -0.052
* Cslculeted from rotational constantsin Table 4 correctedfor the contribution from raaobend using the conversion factor 5.05531 x lo6 emu. Aa.MHz. [6] C. H. TOWNES and A. L. SCHA~LOW,MicrounzveSpctroacopy. McGraw-Hill, New York (1955).
Microwave spectrum of propiolic aaid
1427
0 I.202 124*8 H-C-C I.055
C I.209
I.445 -;I,
124015' 106O19 H
1.343
C 0‘972 Fig. 1. Propiolic &cidmodel structure (8).
in Table 6. The ratio of pa to rubindicates that the dipole moment (l*riQD)makes an angle of N 60’ to the a inertial ~txisof HCGCOOH and is almost parallel to the G=O bond. The dipole moment is slightly lsrger than thtaf in formic acid (141D) [SJ where the dipole is directed between the G-H and C---O bonds at an angle of ~40” to the letter. A similar small increase in the dipole moment, together with a Table 0 AhvjEBMYHz (V/cm)-+ x l@ Observed Calculated
HCCCOOH 101+ 000 321 -312
M=O M=l M=2 M=3 pa = 0.80 f 0.020, &
1.69 1.69 M=3 7-34 7.30 pa = O-76 + 0*02D, ,a* = l-40 + 0.02D il( = 1.69 f 0.03D
+- 101 -312
1.79 2.26 4.20 7.49
Av/E”MHe (volt/cm)-8 x lv Observed Calculated
HCCCOOD 212 321
1.76 2.30 4.22 7.37 = 1.38 * 0*02D
M=O
large change in the direction relative to the G==Obond occurs between formaldehyde (233D) 171and propynal (2.47D) [8]. In propyrml the dipole moment lies between the G-H snd C==O bonds at sn angle of about 34” to the latter. On the basis of the small change in the dipole moment of propynal relative to that of formaldehyde, HOWE and GOLDSTEIN [8] argue against the structure
making a significant contribution to the overall state of the molecule. A similar argument is applicable to propiolic acid. AcknowZedgemrs-D.
G. L. thanks the Science Rasearoh Council for a maintenance grant.
[S] H. KIM, R. KELLAR and W. D. GPIINN,J. Ohms. Fhp. 57,2748 (1962). [7] K. KONDO, J. Phys. Sot. Japan 15, 307 (1960). [8] J. A. HOWE and J. H. GOLDSTEIN, J. Ohm. Phys. 23, 1223 (1956).
&e&o&imica
The conformation and dipole moment of propiolic acid deduced from its microwave spectrum D. G. LISTER end J. K. TYLER Chemistry Department, The University, Glwgow W.2 (Received3 Atqua! 1971) A~Miorowave spectra have been observed for propiolic acid and its two monodeutero species. The moleoule is found to be planar with the hydroxyl proton adopting the expected cis geometry with respect to the oarbony group. A structure consistent with the observed moments of in&k is proposed. The dipole moment hm been mm d ss l-69 A, 0*03D snd is almost parallel to the oarbonyl bond.
acid may be considered to be derived from formic acid by replacing the formyl proton with an acetylene group. One might expect conjugative effects between the carbonyl and acetylenic functions to be revealed in the molecular structure and in the msgnitude and direction of the dipole moment. Further, there exists the formal possibility of internal rotation of the hydroxyl group although it would be anticipated thst the crtrboxylic proton would adopt the planar, cis geometry with respect to the carbonyl group as in formic acid. To investigate this situation we have observed the microwave spectra of propiolic acid and its two monodeuteriated species.
PROPIOLIC
EXPERIMENTAL
All spectra were observed at room temperatures using a conventional 100 kHz. Stark modulation spectrometer. Laboratory grade propiolic acid supplied by KochLight Laboratories, Ltd. was distilled under reduced pressure and used without further purification. HCCCOOD was prepared in the wave guide cell by mixing propiolic acid and D,O vapours. Recordings of regions where HCCCOOD lines were predicted to occur were made with normal propiolic acid and then with the acid/D,0 mixture in the cell. The HCCCOOD lines were identified by comparing the two sets of recordings. DCCCOOH was prepared by the following method. A 50 % aqueous solution of propiolic acid was neutralized with dilute sodium hydroxide solution st 0°C. The water was removed under reduced pressure at room tempersture and the residual sodium propiolate (1 g) dissolved in D,O (3 ml). The pH of the solution wss sdjusted to 12 by adding a few drops of a solution of sodium deuteroxide in D,O and the mixture allowed to stand for two days et room temperature. After removing the water under reduced pressure the residue was dissolved in Hz0 (3 ml). The solution was cooled to O”C, a few drops of concentrated hydrochloric acid added and the propiolic acid extracted into ether. The ethereal solution was dried with anhydrous magnesium sulphate, filtered and the ether removed under reduced pressure. 1423
D. G. LISTER and J. K. TYLER
1424
ANALYSIS
OF SPECTRA
the purposes of predicting spectra a model for propiolic acid was set up based on the accurate data available for propynal [l] and formic acid [2]. This suggested that propiolic acid is a near prolate asymmetric rotor with KN -03, having an appreciable ,u~component of the dipole moment and a smaller ,u,,component. The strongest, readily accessible transitions were predicted to be the ,u,,, J,,+, f- J,,J and Jz,J_a c J1,J_l, Q-branch series together with the p,,, R-branch and a few pa, R-branch lines. Initial assignments followed the usual procedure of resolving Stark effects and making Q- and R-branch plots. Attempts to fit line frequencies to rigid rotor theory revealed the need to allow for a small amount of centrifugal distortion in the higher J, Q-branch lines. A least-squares fitting technique to (A + C)/2, (A - C)/2, K Tag Tbbbb* r~~cc and *dab was adopted using expressions for the centrifugal distortion energy in the form given by SORENSEN [3]. The expectation values of the quartic angular momenta terms were calculated from the formulae of WATSON [4]. A comparison of the observed and calculated line frequencies is given in Tables l-3 and the Par
Table 1. Line frequencies for HCCCOOH
Oberved 111+ OcKl %s+
101
211+
11,
41s +
31,
4,s +
38,
308 + 20, 31s+ 20, %1+ 21% 308 + 21, 31,+- 31, %o+- 21, 31,+ 211 3m+ %I 3II + %o 404 i-. 30, 401- 31, 3,1+- 31, 428 +
3,l
48, +
331
4a1+
3*0
41s +
404
4,s +
41,
514 +
50.5
5,s +
514
324 +
616
%s + 8 96 +
71,
%7 +
918
81,
16194.60 21363.44 16626.22 21302.43 27041.23 27076.23 14302.49 20041.03 23988.46 23222.36 21693.73 22086.47 27979.80 22241.36 22851.45 30832.76 28847.66 29792.68 29106.63 29141.26 14969.83 21810*89 18886*81 21199.49 21320.67 22416.27 24667.72 28196.47
(MHz)
Rigid rotor
Centrifugal distortion ensrgy
Celwleted
16194.69 21303.67 16626.32 21302.47 27041.28 27076.78 14302.40 20041.20 23988.81 23222.70 21694.33 22086.18 27980.32 22241.61 22862.29 30833.42 28848.31 29793.47 29108.17 29143.01 14960.74 21812.34 18888*10 21201.48 21323.56 22419.86 24672.32 28201.73
-0.04 -0.11 -0.16 -0.08 -0.22 -0.66 0.00 -0.14 -0.64 -0.31 -0.68 -0.64 -0.18 -0.05 -0.87 -0.64 -0.86 -0.99 - 1.73 - I.74 -0.79 - I.33 -1.29> - 1.92 -2.66 -3.69 -4.G -6.20
16194.66 21363.47 16626.16 21302.40 27041.06 27076.12 14302.40 20041.07 23988.27 23222.40 21693.76 22086.64 27980.14 22241.47 22851.42 30932.88 28847.46 29792.48 29106.44 29141.27 14959.96 21811.01 18886.81 21199.67 21320.90 22416.26 24667.67 28196.63
C. C. COSTAIN ELnd J. R. MORTON, J. Chem. Phys. 31, 389 (1959). [2] G. H. KWEI EtndR. F. CURL, J. Chem. phys. 35,1592 (1960). [3] G. 0. SORENSEN, J. Mol. Spectry 22, 325 (1967). [4] J. K. G. WATSON,J. Chem. Phy8. 46, 1935 (1967). [I]
Microwave spectrum of propiolic acid
1425
Table 2. Line frequencies for HCCCOOD (MHz)
Observed 214+
101
3,s + 2,1+
20, 21,
3111+ 21, 2ao'211 31,+211 4Id-+ 30.9 4Ol+- 31, 414- 318 3!u+ 31, 4s*+-381 481+- 330 4as+ 418 6!d11+614 6IS+--%, %4+ %4 7 14 -707 'SK +
'14
‘%e+%, %,-914
20546*10 20666.15 26655.00 19447.59 23621.02 22602.56 31630.83 21469.46 26533.65 22619.44 25216.40 25247.20 21497.66 20871.06 23099.66 20931.01 25566*6’ 2191oJ79 23957.25 2’251*69
Rigid rotor
20546.10 20666~10 26655.26 19447~50 23621.11 22602.56 31630.54 21649.10 26533.43 22619.76 25216.35 25245.25 21495.36 205'2.41 23100.96 20933.14 25565.49 21913.75 23991.26 2’256.63
Centrifugal distortion energy
+0*01 $-OS13 -0.20 +0*05 -0.0' -0.09 +0*14 +0*37 +0*22 -0.3' -1.05 -1.09 -0.50 -1.35 -1.43 -2.09 -1.50 -2.93 -3.55 -4.99
Calculated
20546.04 20666~23 26658*06 19447.5' 23621.04 22602.7' 31630.95 21469.63 26533.64 22619.39 25216.30 25247.30 21497.66 205'1.03 23099.63 20931.06 2556&69 21910.56 23957.37 2’25143
derived constants are given in Table 4. The rotational constants are well determined from the transitions studied, but little reliance can be placed on the centrifugal distortion constants (e.g. raabaand rcccc for HCCCOOD have both apparently positive values). This situation is largely due to the small centrifugal corrections involved in these particular transitions. A number of weaker vibrational satellites are observed to accompany each ground state transition, but no accurate measurements have been made. No evidence for other than a single rotameric form of propiolic acid has been found in this work. Table 3. Line frequencies for DCCCOOH (MHz)
Observed 21, +
303 313
101
+-202 +202
322 +221
321 +220 404 + 303 4 04 +
313
3 21 -
312
413 +
312
4 32 -
331
523 - %4 6 24 +%s '25 +
'16
826 +
817
92, +
913
20808.69
19871.11 26176.60 20167.31 20443.84 26178.12 19873.04 23896.40 28660.70 27010.11 22076.74 21821.64 2230557 23699.09 26131.11
Centrifugal distortion energy
Calculated
20808.80 19871.29 26176.82 20158~00 20444.70
-0*13 -0.19 -0.33 -0.69 -0.72
20808.67 19871*10 27176.49 20157.31 20443.98
19873*21 23896.85 28561.44 27012.02 22078.22 21823.70 22308.43 23702.77 26136.94
-0.30 -0.46 -0.74 -1.96 -1.45 -2.01 -2.86 -3.75 -4.80
19872.91 23896.39 28560.69 27010.06 22076.78 21821.60 22306.57 23699.02 26131.13
Rigid rotor
1426
D. G. LISW
and J. K. TYLEB
Table 4. Rotstionsl oonstents and centrifugal distortion constants (MHz)
A B
0 rluzaa ?bbb
Tcccc 7abab
HCCCOOH
HCCCOOD
DCCCOOH
12110~09 4146.94 3034.49 -0.1476 -0.0076 -0*0025 -0.0426
11858.32 401569 2995.58 -0.0172 $0~0013 +0.002s -0.0563
12109.93 3819.71 289963 -0.0355 -0~0101 -0*0060 -0.0160
CONFORMATION AND STRUCTURE
The moments of inertia and the inertial defects of the three isotopic species of propiolic acid are given in Table 5. The positive values for the inertial defects indicate that the molecule is essentially planar. The substitution co-ordin&es of the hydrogen atoms in the principal rtxis system of HCCCOOH were calculated from the Ib and I, values and are comprtred in Table 6 with these parameters calculsted from the structure given in Fig. 1. This structure is based on the accur&ely known data for propynal [l] and formic acid [2] and reproduces the observed moments of inertia quite closely. There is no doubt that the hydroxyl proton adopts the cis configuration with respect to the carbonyl group and we believe that the geometry shown in Fig. 1 is a reasonable approximation to the structure of propiolic acid in the gas phase. DIPOLE MOMENT
Stark effect measurements were made on the l,, c O,, and 3,, t 3,, transitions of HCCCOOH and the 2,, t l,, and 3,, c 3,s transitions of HCCCOOD. The 2 + 1 transition of OCS was used for calibration taking the dipole moment of this molecule as 0.7124D. All Stark displacements studied in propiolic acid were found to be proportional to E2 and the usual second order perturbation treatment [5] is adequate in this case. For both HCCCOOH and HCCCOOD ,u,,and ,ubwere obtained by graphically solving the appropriate simultaneous equations and the results are summarized Table 5. Moments of inertia snd hydrogen atom aoordinates
43 HCCCOOH* HCCCOOD* DCCCOOH* Model structure
Ib
41.7447 42.6308 41.7452 42.1126
121.9052 152*8897 132.3484 120.9109
Hydroxyl H Observed Model structure
IO 1636929 168.7590 174.3432 163.0235
A (a.m.u. A) 0.2430 0.2369 0.2496 -
Acetylenic H
o
b
a
b (A)
1.993 1.963
-0.968 -1.022
-3.244 -3.232
-0.024 -0.052
* Cslculeted from rotational constantsin Table 4 correctedfor the contribution from raaobend using the conversion factor 5.05531 x lo6 emu. Aa.MHz. [6] C. H. TOWNES and A. L. SCHA~LOW,MicrounzveSpctroacopy. McGraw-Hill, New York (1955).
Microwave spectrum of propiolic aaid
1427
0 I.202 124*8 H-C-C I.055
C I.209
I.445 -;I,
124015' 106O19 H
1.343
C 0‘972 Fig. 1. Propiolic &cidmodel structure (8).
in Table 6. The ratio of pa to rubindicates that the dipole moment (l*riQD)makes an angle of N 60’ to the a inertial ~txisof HCGCOOH and is almost parallel to the G=O bond. The dipole moment is slightly lsrger than thtaf in formic acid (141D) [SJ where the dipole is directed between the G-H and C---O bonds at an angle of ~40” to the letter. A similar small increase in the dipole moment, together with a Table 0 AhvjEBMYHz (V/cm)-+ x l@ Observed Calculated
HCCCOOH 101+ 000 321 -312
M=O M=l M=2 M=3 pa = 0.80 f 0.020, &
1.69 1.69 M=3 7-34 7.30 pa = O-76 + 0*02D, ,a* = l-40 + 0.02D il( = 1.69 f 0.03D
+- 101 -312
1.79 2.26 4.20 7.49
Av/E”MHe (volt/cm)-8 x lv Observed Calculated
HCCCOOD 212 321
1.76 2.30 4.22 7.37 = 1.38 * 0*02D
M=O
large change in the direction relative to the G==Obond occurs between formaldehyde (233D) 171and propynal (2.47D) [8]. In propyrml the dipole moment lies between the G-H snd C==O bonds at sn angle of about 34” to the latter. On the basis of the small change in the dipole moment of propynal relative to that of formaldehyde, HOWE and GOLDSTEIN [8] argue against the structure
making a significant contribution to the overall state of the molecule. A similar argument is applicable to propiolic acid. AcknowZedgemrs-D.
G. L. thanks the Science Rasearoh Council for a maintenance grant.
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