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Microwave spectrum and dipole moment of glycolic acid
Volume 82, number 3
15 September 1981
CHEMICAL PHYSICS LETTERS
MICROWAVE SPECTRUM AND DIPOLE MOMENT OF GLYCOLIC
ACID
C.E. BLOM and A. BAUDER Laboratory of Physical Chenmtry, Swiss Federal Instuute of Technology, CH-8092 Zurich. Switzertaand Received 6 May 198 1
More than 200 rotational transitions of glycolic acid (CH20HCOOH) have been assigned in the frequency range 18-40 GHz. Rotational constants and centrifugal distortion constants for the ground state and two viirationally excited states were deduced. From Stark splittings the dipole moment was determined: ua = 1.913(s). ,.Q = 0.995(14) and flto~ = 3 156(g) D
1. Introduction Our interest in glycolic acid arose during investigations into the properties of similar molecules with intramolecular hydrogen bonds. Glycolic acid might exist in more than one ccnformation. Ha et al. [l] concluded from ab initio quantum-chemical calculations that the lowestenergy conformer exhibits a pIanar skeieton with an intramolecular hydrogen bond forming a five-membered ring. The hydroxyl group points towards the carbonyl oxygen of the carbonyl group in its standard conformation_ This result is consistent with experimental data of the crystal structure from X-ray and neutron diffraction [2,3] and from rotational spectra of the free molecule. Sharpen [4] reported a preliminary study of the microwave spectrum of glycolic acid, but gave no pertinent data except the rotational constants for the g-round vibrational state. We hhve investigated the microwave spectrum of glycolic acid betw xen 18 and 40 GHz. Roiational transitions of the gr2uzd state and of two excited states of the lowest out-of-plane vibration v21 have been assigned. The analysis included higher J transitions and enabled us to determine centrifugal distortion constants. A careful search did not reveal any transition of another possi&le conformer.
492
2. Experimental
details
A commercial sampIe of glycolic acid (Fluka, content > 99%) was purified by sublimation. The microwave spectra were recorded over the frequency range from 18 to 40 GHz on a conventional Stark spectrometer with 30 kHz modulation. Because of the low vapour pressure of glycolic acid, sample and inlet cubes were heated to =SOOC whereas the Stark cell was kept at 52°C. A small flow of the substance at pressures between 15 and 20 mTorr through the 2 m Stark cell improved the stability of the spectral recordings. For signal enhancement, digital filtering and frequency measurements, the spectrometer could be connected to a PDPS/E computer. The accuracy of the frequency measurements is estimated to be better than 40 kHz.
3. Assignments and analysis With the help of the preliminary rotational constants reported by Scharpen 141 the 1owJ R-branch transitions of the ground vibrational state of glycolic acid could easily be assigned. Considering centrifugal distortion corrections, higher J values of R- and Qbranch transitions were included step by step. This procedure fmally led to the assignment of 97 rotational transitions for the ground state as shown in 0 009-2614/81/0000-0000/$02.75
0 1981 North-Holland
Volume 82,number3
CHEMICALPHYSICSLETTERS
1.5September1982
Table1 Table1 (continued) Measuredrotational transition frequencies(inMHz)ofglycolicacidinthevibrationalgroundstateandintwoviirationalTransItion Groundstate u21=l u21=2 lyexcited states ofvzl(a") 8 ; 22810.601 1,7-'2,6 22605.893 22409.082 8 3,5_82,6 28204.990 TransitionGround state 021-l Ll21=2 22342.831 21912.939 26953.011 36255 316 26467.299 27202054 26981 776 35082.751 30089.315 20681.255 32311.738 21592.671 31721074 21582.431 21574.223 27204.949 26977.954 19483.425 26757.815 19493.690 19505.007 28481403 28234399 27996020 22644.401 22624.997 22607.861 29369.815 21136.930 21131.581 21127.971 20981.262 20759.931 25188.341 25153.273 25120.386 31196.935 30915571 18999.533 30645.338 28161.969 24757.622 27595.385 35399.540 35069.940 27096.520 34754.689 27100657 27106.483 36233.600 35945.034 29177529 29159.969 29145321 23845.553 25859.739 25874.649 25890882 25715.664 28392.497 28383.154 35807.439 30037.198 30012.874 29991.645 36094.430 35728.109 35381099 28440.285 28430.417 28422937 35723 890 35429290 35143.237 28091.801 28085676 28081.674 31498.692 30366.790 30347.207 35013.922 22589.312 22627997 18186.838 18139.836 36650.391 26989.762 36334.152 36028.813 24313.141 33234992 33260.020 25244.349 36959.349 24963.909 24701016 36932.699 36909.850 33570.039 32155.861 32176.142 32198230 33337.150 32976.444 35549.774 35528202 32519-459 37272724 37245 284 37221.510 32960.323 35714.452 35700.175 35689.222 21216.172 34970007 20960 590 20722822 34963.824 34960.371 29053.449 28714.580 28398271 35426.187 35422.804 23923.667 23620.207 23339351 29964.686 29999.952 32476.389 32079299 31995094 18650.092 18391.748 33388.747 33119.065 32856.320 26347.143 25996.079 25672.799 29803.930 29301.813 35558.441 35103.390 34682.666 36814.986 36510.049 20311.070 20015.341 19745.306 38373.731 28514.250 28116.025 27750.907 30106.291 30186.905 21785.997 21453,464 37001.023 37036.319 37073.432 30446.380 30001.470 29595.421 22909.172 23088.947 22720.243 18360.479 18205.721 19055.966 32160.916 31670002 31223.962 31706.256 31452.582 31205.139 24231.833 23827.863 33185.053 32897.821 33672.478 37772.016 25225 l55 34261883 34993.571 29847.635 29610.318 29378.718 18167.119 19968.382 19794.805 19627.271 26078.096 37095.137 36771.177 18708.844 31952.893 34812.814
493
Volume 82, number 3
CHEhlICAL PHYSICS LETTERS
15 September 1981
Table 2
Constants (ii MHZ), centrifugal distortion constants (in kHz) and inertia defect (ii u A*) for glycdic acid in the viirational sound state and two excited states
Rotational
__-.
Ground state
rJ*1 =2
u*1 = 1
10578.2469(50) 10636.1914(41) 4045.1649(26) 4039.8048(29) 2998.7414(26) 3002.8843(29) O-72.56(631) 0 8629 (537) AJ 3.352(69) 3.464(82) 4.988(454) 4.474(552) zh-c O-2119(23) O-2132(28) 6-J 2.435(62) 2.256(76) 6K -7*72 --3.--PA -3.9187 4.5773 n a) 85 53 44 b) 30 J 30 30 0.033 0.032 am@%z) ‘1 0.033 ___ ________ -____- __-________ a) Number of measured rotational transrtions included in least-squares fit. b) Highest value of J in fit. c) >Iean residual error of a measured transition frequency. A B C
10696.0950(24) 4051.0323(S) 2994.6632(S) O-8162(57) 3.276(29) 5 602(96) 0.2119(15) 2 581(40)
table I_ Above J = 38 deviations from quartic centrifugal distortion corrections became progressively noticeable_ Therefore in the final adjustment of the rotational constants and the quartic centrifugal dis-
tortion constants only 8.5 transitions with J < 30 were considered. The results together with the standard deviations are collected in table 2. The centrifugal distortion constants are defmed with respect to Watson’s asymmetric reduction in a prolate Ir representation [5]. AU five centrifugal distortion constants are well determined from the data. The mean residual deviation of a measured transition frequency of 33 kHz is compatrble with the estimated accuracy of the frequency measurements_ The largest deviations do not exceed 110 kHz. For transitions with J > 30 predicted frequencies with the constants in table 2 deviate with increasing J from the measured frequencies but remain witbin 1 MHz. For the 3 + 2 pa R-branch transitions the corresponding transitions of the excited vrbrational states (taco = 1,2) fell within 40 MHz from the groundstate transitions. They were readily assigned although occasionally some highJ Q-branch transitions interfered. A similar procedure as for the ground state was followed for additional assignments. Measured transition frequencies of the two excited states are included in table 1_ Adjusted rotational constants and centrifugal distortion constants are shown in table 2. From relative peak intensities the energy separation 494
between the ground and first excited state “zl(a”) was estimated to be 123 + 15 cm-l _
4. Electric dipole moment The Stark shifts of different hl components have been measured for the transition 41,3-31 2 and 52 4, , Table 3 hIeasured and cahhted Stark coeffhzients (in Hz Ve2 cm*) and electric dipole moment (in D) of glycolic acid Transition
M
AViE obs. a)
413-312
524 -423
3 2 1
-7.744(38) 4_488(26) -2.381(20)
4 3 2
67.73(55) 37.228(86) 15.265(24)
I~J = l-913(5) b) I&l = O-995(14)
CdC.
b)
-7.925 4.398 -2 282 68 594 37.458 15.219
I&i = 0.0 c) htoti = 2.156(g)
a) A dipole moment of 0.71521 D for OCS [6] was used to calibrate the Stark cell. b) The uncertainty is given in parenthesesas one standard deviation. C) Assuming a planar frame.
Volume 82, number 3
CHEMICAL PHYSICS LElTERS
42,3. The non-vanishing components of the permanent electric dipole moment were determined in a least-squares fit of measured and calculated Stark slopes. A symmetry plane in the planar skeleton has been assumed a priori. The results with their standard deviations are given in table 3. The experimental dipole moment agrees well with that from a recent ab initio calculation [l] , where the following components pa = 2.526, pb = 0.798 and ptoti = 2.631 D were found.
1.5 September 1981
References [l] T.-K. Ha, CE. Blom and Hs.H Giinthard, J. Mol. Struct (1981), to be published. [Z] R.D. EBison, C.K. Johnson and H.A. Levy, Acta Cryst B27 (1971) 333. [3] W.P. Pijper, Acta Cryst. B27 (1971) 344. 143 L.H. Scharpen, Abstracts of the 27th Symposium on hfolecular Structure and Spectroscopy, Columbus, Ohio, E4 (1972) p_ 77. [5 ] J.K.G. Watson, in: Vibrational spectra and structure, Vol. 6, ed. J-R. Durig (Elsevier, Amsterdam, 1977) pp. l[6] ?k
Muenter, J. Chem Phys. 48 (1968) 4544.
495
15 September 1981
CHEMICAL PHYSICS LETTERS
MICROWAVE SPECTRUM AND DIPOLE MOMENT OF GLYCOLIC
ACID
C.E. BLOM and A. BAUDER Laboratory of Physical Chenmtry, Swiss Federal Instuute of Technology, CH-8092 Zurich. Switzertaand Received 6 May 198 1
More than 200 rotational transitions of glycolic acid (CH20HCOOH) have been assigned in the frequency range 18-40 GHz. Rotational constants and centrifugal distortion constants for the ground state and two viirationally excited states were deduced. From Stark splittings the dipole moment was determined: ua = 1.913(s). ,.Q = 0.995(14) and flto~ = 3 156(g) D
1. Introduction Our interest in glycolic acid arose during investigations into the properties of similar molecules with intramolecular hydrogen bonds. Glycolic acid might exist in more than one ccnformation. Ha et al. [l] concluded from ab initio quantum-chemical calculations that the lowestenergy conformer exhibits a pIanar skeieton with an intramolecular hydrogen bond forming a five-membered ring. The hydroxyl group points towards the carbonyl oxygen of the carbonyl group in its standard conformation_ This result is consistent with experimental data of the crystal structure from X-ray and neutron diffraction [2,3] and from rotational spectra of the free molecule. Sharpen [4] reported a preliminary study of the microwave spectrum of glycolic acid, but gave no pertinent data except the rotational constants for the g-round vibrational state. We hhve investigated the microwave spectrum of glycolic acid betw xen 18 and 40 GHz. Roiational transitions of the gr2uzd state and of two excited states of the lowest out-of-plane vibration v21 have been assigned. The analysis included higher J transitions and enabled us to determine centrifugal distortion constants. A careful search did not reveal any transition of another possi&le conformer.
492
2. Experimental
details
A commercial sampIe of glycolic acid (Fluka, content > 99%) was purified by sublimation. The microwave spectra were recorded over the frequency range from 18 to 40 GHz on a conventional Stark spectrometer with 30 kHz modulation. Because of the low vapour pressure of glycolic acid, sample and inlet cubes were heated to =SOOC whereas the Stark cell was kept at 52°C. A small flow of the substance at pressures between 15 and 20 mTorr through the 2 m Stark cell improved the stability of the spectral recordings. For signal enhancement, digital filtering and frequency measurements, the spectrometer could be connected to a PDPS/E computer. The accuracy of the frequency measurements is estimated to be better than 40 kHz.
3. Assignments and analysis With the help of the preliminary rotational constants reported by Scharpen 141 the 1owJ R-branch transitions of the ground vibrational state of glycolic acid could easily be assigned. Considering centrifugal distortion corrections, higher J values of R- and Qbranch transitions were included step by step. This procedure fmally led to the assignment of 97 rotational transitions for the ground state as shown in 0 009-2614/81/0000-0000/$02.75
0 1981 North-Holland
Volume 82,number3
CHEMICALPHYSICSLETTERS
1.5September1982
Table1 Table1 (continued) Measuredrotational transition frequencies(inMHz)ofglycolicacidinthevibrationalgroundstateandintwoviirationalTransItion Groundstate u21=l u21=2 lyexcited states ofvzl(a") 8 ; 22810.601 1,7-'2,6 22605.893 22409.082 8 3,5_82,6 28204.990 TransitionGround state 021-l Ll21=2 22342.831 21912.939 26953.011 36255 316 26467.299 27202054 26981 776 35082.751 30089.315 20681.255 32311.738 21592.671 31721074 21582.431 21574.223 27204.949 26977.954 19483.425 26757.815 19493.690 19505.007 28481403 28234399 27996020 22644.401 22624.997 22607.861 29369.815 21136.930 21131.581 21127.971 20981.262 20759.931 25188.341 25153.273 25120.386 31196.935 30915571 18999.533 30645.338 28161.969 24757.622 27595.385 35399.540 35069.940 27096.520 34754.689 27100657 27106.483 36233.600 35945.034 29177529 29159.969 29145321 23845.553 25859.739 25874.649 25890882 25715.664 28392.497 28383.154 35807.439 30037.198 30012.874 29991.645 36094.430 35728.109 35381099 28440.285 28430.417 28422937 35723 890 35429290 35143.237 28091.801 28085676 28081.674 31498.692 30366.790 30347.207 35013.922 22589.312 22627997 18186.838 18139.836 36650.391 26989.762 36334.152 36028.813 24313.141 33234992 33260.020 25244.349 36959.349 24963.909 24701016 36932.699 36909.850 33570.039 32155.861 32176.142 32198230 33337.150 32976.444 35549.774 35528202 32519-459 37272724 37245 284 37221.510 32960.323 35714.452 35700.175 35689.222 21216.172 34970007 20960 590 20722822 34963.824 34960.371 29053.449 28714.580 28398271 35426.187 35422.804 23923.667 23620.207 23339351 29964.686 29999.952 32476.389 32079299 31995094 18650.092 18391.748 33388.747 33119.065 32856.320 26347.143 25996.079 25672.799 29803.930 29301.813 35558.441 35103.390 34682.666 36814.986 36510.049 20311.070 20015.341 19745.306 38373.731 28514.250 28116.025 27750.907 30106.291 30186.905 21785.997 21453,464 37001.023 37036.319 37073.432 30446.380 30001.470 29595.421 22909.172 23088.947 22720.243 18360.479 18205.721 19055.966 32160.916 31670002 31223.962 31706.256 31452.582 31205.139 24231.833 23827.863 33185.053 32897.821 33672.478 37772.016 25225 l55 34261883 34993.571 29847.635 29610.318 29378.718 18167.119 19968.382 19794.805 19627.271 26078.096 37095.137 36771.177 18708.844 31952.893 34812.814
493
Volume 82, number 3
CHEhlICAL PHYSICS LETTERS
15 September 1981
Table 2
Constants (ii MHZ), centrifugal distortion constants (in kHz) and inertia defect (ii u A*) for glycdic acid in the viirational sound state and two excited states
Rotational
__-.
Ground state
rJ*1 =2
u*1 = 1
10578.2469(50) 10636.1914(41) 4045.1649(26) 4039.8048(29) 2998.7414(26) 3002.8843(29) O-72.56(631) 0 8629 (537) AJ 3.352(69) 3.464(82) 4.988(454) 4.474(552) zh-c O-2119(23) O-2132(28) 6-J 2.435(62) 2.256(76) 6K -7*72 --3.--PA -3.9187 4.5773 n a) 85 53 44 b) 30 J 30 30 0.033 0.032 am@%z) ‘1 0.033 ___ ________ -____- __-________ a) Number of measured rotational transrtions included in least-squares fit. b) Highest value of J in fit. c) >Iean residual error of a measured transition frequency. A B C
10696.0950(24) 4051.0323(S) 2994.6632(S) O-8162(57) 3.276(29) 5 602(96) 0.2119(15) 2 581(40)
table I_ Above J = 38 deviations from quartic centrifugal distortion corrections became progressively noticeable_ Therefore in the final adjustment of the rotational constants and the quartic centrifugal dis-
tortion constants only 8.5 transitions with J < 30 were considered. The results together with the standard deviations are collected in table 2. The centrifugal distortion constants are defmed with respect to Watson’s asymmetric reduction in a prolate Ir representation [5]. AU five centrifugal distortion constants are well determined from the data. The mean residual deviation of a measured transition frequency of 33 kHz is compatrble with the estimated accuracy of the frequency measurements_ The largest deviations do not exceed 110 kHz. For transitions with J > 30 predicted frequencies with the constants in table 2 deviate with increasing J from the measured frequencies but remain witbin 1 MHz. For the 3 + 2 pa R-branch transitions the corresponding transitions of the excited vrbrational states (taco = 1,2) fell within 40 MHz from the groundstate transitions. They were readily assigned although occasionally some highJ Q-branch transitions interfered. A similar procedure as for the ground state was followed for additional assignments. Measured transition frequencies of the two excited states are included in table 1_ Adjusted rotational constants and centrifugal distortion constants are shown in table 2. From relative peak intensities the energy separation 494
between the ground and first excited state “zl(a”) was estimated to be 123 + 15 cm-l _
4. Electric dipole moment The Stark shifts of different hl components have been measured for the transition 41,3-31 2 and 52 4, , Table 3 hIeasured and cahhted Stark coeffhzients (in Hz Ve2 cm*) and electric dipole moment (in D) of glycolic acid Transition
M
AViE obs. a)
413-312
524 -423
3 2 1
-7.744(38) 4_488(26) -2.381(20)
4 3 2
67.73(55) 37.228(86) 15.265(24)
I~J = l-913(5) b) I&l = O-995(14)
CdC.
b)
-7.925 4.398 -2 282 68 594 37.458 15.219
I&i = 0.0 c) htoti = 2.156(g)
a) A dipole moment of 0.71521 D for OCS [6] was used to calibrate the Stark cell. b) The uncertainty is given in parenthesesas one standard deviation. C) Assuming a planar frame.
Volume 82, number 3
CHEMICAL PHYSICS LElTERS
42,3. The non-vanishing components of the permanent electric dipole moment were determined in a least-squares fit of measured and calculated Stark slopes. A symmetry plane in the planar skeleton has been assumed a priori. The results with their standard deviations are given in table 3. The experimental dipole moment agrees well with that from a recent ab initio calculation [l] , where the following components pa = 2.526, pb = 0.798 and ptoti = 2.631 D were found.
1.5 September 1981
References [l] T.-K. Ha, CE. Blom and Hs.H Giinthard, J. Mol. Struct (1981), to be published. [Z] R.D. EBison, C.K. Johnson and H.A. Levy, Acta Cryst B27 (1971) 333. [3] W.P. Pijper, Acta Cryst. B27 (1971) 344. 143 L.H. Scharpen, Abstracts of the 27th Symposium on hfolecular Structure and Spectroscopy, Columbus, Ohio, E4 (1972) p_ 77. [5 ] J.K.G. Watson, in: Vibrational spectra and structure, Vol. 6, ed. J-R. Durig (Elsevier, Amsterdam, 1977) pp. l[6] ?k
Muenter, J. Chem Phys. 48 (1968) 4544.
495