March 20, 1957
SPECTRA OF F O R M A M I D E - AND N-~~ OF FORMAMIDE
[ CONTRIBUTIOS FROM THE DEPARTMENT OF CHEMISTRY O F LOUISIANA POLYTECHNIC
1349
INSTITUTE]
Raman Spectra of Formamide-N-d, and of Formamide in Concentrated Hydrochloric Acid BY CHARLES H. SIVIITH'.~ AND J. DONALD ROBINSON RECEIVED AUGUST6, 1956 The Ranian spectra of HCONDz and of HCONH, in concentrated hydrochloric acid have been run. A partial ititerpretation of the spectra is given. It is shown t h a t there is a considerable amount of coupling between the various types of vibrations so t h a t it is sometimes difficult to pick out corresponding lines in HCONHz and HCOXDz. The spectrum in concentrated HC1 exhibits an unusually diffuse and double band in the Y( C-H) region. This is interpreted as due to Fermi resonance between v(C-H) and the stretching vibrations of hydrogen in -?;Ha+.
Introduction The interpretation of the spectrum of formamidc is of particular importance since it is the simplest of the amides, and data from formamidc are important in extending the interpretation to more complicated amides and peptides. The Raman spectrum of formamide has been studied extensively by the Graz group,3by Rao4and by S a k ~ e n a . ~ The infrarared spectrum of formamide has been studied by Lecomte and Freymann,'j by Evans7v8and by Miyaz a ~ a the , ~ latter also having studied the infrared spectrum of formamide-N-d2, a few lines of which had been found by Evans.' The Raman spectrum of formamide-N-d2is reported for the first time, as is the Raman spectrum of formamide in concentrated hydrochloric acid, Experimental Methods The Rainan spectrum of formamide in coticeiitratcd hydrochloric acid was run photographically on the Gaertner spectrograph at the University of Michigan using the 4358 A . Hg-line of a Toronto arc as the exciting line and the argon lines for frequency determination. T h e Gaertner spectrograph gave a dispersion of 213 cm.-I per mm. a t 4700 A. wave length. T h e powerful light source enabled exposure times of 5 minutes to 1 hour t o be used. The Raman spectrum of formamide-S-dz was run photographically in our own laboratories using a n Applied Research Laboratories Rnman Spectrograph, the 4358 A. Hg-line of General Electric Type H-11 lamps as the exciting line and the lines of an iron arc for wave number measurement. T h e Applied Research Laboratories spectTograph gave a dispersion of 105 cm.-' per Inm. at 4700 A. wave length. The weak light source made exposure times of 1 t o 24 hours necessary. Because of rapid hydrolysis of formamide by concentrated hydrochloric acid i t was necessary to run this Raman spectrum a t -9' and to give exposure times of less than 30 minutes. Under tlicse conditions no appreciable hydrolysis took place since the Raman bands of formic acid could not be detected. For comparison purposes the spectrum of formic acid in concentrated hydrochloric acid also was run T h e infrared spectra of formamide and formaa t -9". niide-h--d2 have been run on a Beckmaii IR-2 infrared spectrophotometer usiiig a S a C l prism and NaCl cells. In our infrared measurements we actually had a saturated solution (1) We wish to acknowledge receipt of a Frederick Gardner Cottrell Grant from Research Corporation making t h e research possible. ( 2 ) We appreciale cooperatiun from t h e Chemistry Department of t h e University of Michigan where a portion of this work was done. (3) L k s t completely b y I,. Kahovec a n d H . W;i\smuth, L . physik. C h m , B48, 70 (1940). (4) A . I>. S. Rao, J . l i i d i a n C h c m . S u c . , 18,337 (1941) ( 5 ) B. D. Saksena, PYOC. Indiuii Acacf. Sei., llA, 5 3 (1940). ( 6 ) J. 1,ecomte and R. I"reymann, Rzill. S U C . iiiiin., 8 , G I 2 (1041). (7) J. C . Evans, J. Chem. Phrs., 22, I ? % (19%). (8) J. C . Evans, J . p h y s . radium, 15, 314 (1954). (9) T. Rliyazawa, J. Chem. SOC.J a p a n . Piwe Chem. Sect., 76, 81'1 (1955).
of KaC1 in formainide. Uuriug the infrared mcasuremelits it was found necessary to keep the formamide-N-dz film from exposure t o the air, since rapid exchange with atmospheric moisture took place and we would then obtaiu the spectrum of ordinary formamide. Materials.-Formaniide was prepared by the amnionolyais of ethyl formate, several vacuum distillations, fractional freezing and a final vacuum distillation in an all-glass apparatus free from ground glass joints. A commercial product also was purified and found t o give a spectrum identical with the prepared formamide. It was found necessary to purify the formamide immediately before use because of its teridency t o darken on standing. Formamide-N-dz was prepared from purified formamide by mixing the formamide with a n equal volume of D?O,'O holding a t 100" for several hours and removing the water by vacuum distillation. This procedure was repeated four times and the formamide-N-dz was purified by three vacuum distillations, the last in a n all-glass apparatus free from ground glass joints. It was necessary t o separate a small amount of ammonium formate during the vacuum distillation. This fact, along with our subsequent observation of the rapidity of isotope exchange in this case, makes us suggest to subsequent workers t h a t the exchange should be done more rapidly and at a lower temperature. Because of the danger of isotopic exchange, no attempt was made t o purify the formamide-N-dz by fractional freezing. The Raman lines of ordinary formamide could not be detected in the Raman spectrum of forniamide-X-&. IZcagent grade concentrated hydrochloric acid was used without purificcition.
Results Our infrared wave numbers were accurate only in the 700 to 1800 cm.-' region because we were limited to a NaCl prism. In this region for liquid formamide our results are in agreement with those of Evans' and will not be relisted. In the region 700 to 1800 cm.-l our measurements on the infrared spectrum of formamide-N-d2 agree very closely with those of Miyazawag except that we did not detect the weak band observed by Miyazawa a t 1493 cm.-l and we observed a very, very weak band a t 762 an.-' rather than the 710 cm.-' reported by Miyazawag (750 cm.-' by Evans'). It is also of interest to note that our 1112 cm.-' band is of much smaller intensity than the one reported by Miyazawa a t 1115 cm.-I. Our Raman results are given in Table I. The failure t o observe bands in the 600 cm.-' region and in the 2400 cm.-l region in HCOT\TD2 is presumably due to their extreme weakness rather than to their absence. The most striking feature in the spectrum of formamide in concentrated hydrochloric acid is the broadening of all bands and the extreme broadening and doubling of the 2900 C I I L - ~ band. This effect is not observed in the spectrum of formic acid in HCI. (10) T h e D,O was eCl.790 from Stuart Oxygcn Ccirnpdnq
the Raman spectrum while the l(iO0 cm-I band The 1115 c m - ' hand in HCONDz might also correspond to the 1093 ern.-' TIES'~ HCOOH in HCOb-Hz band in HCONH2. A similar situation was obHCONHz HCOhTHz in HCONDz concd HCA 257' inn concd. HC; liquid liquid served by Otvos and EdsalllGin the Raman spectra (25%) -9 250 (25%) -9 35' Hz025 of CO(NH2)2 and CO(ND2)2. In these compounds, the 1604 (4b) cm.-' band disappears on deuteration 600(2) and bands appear a t 1201 ( l b ) ern.-' and 1164(1) 109 l(4) 914(2) ern.-'. The 1164(1) cm.-I band in CO(ND2)J 1120(1) might be interpreted as arising from the 1167(4) 1332(4) 1345(6> ern.-' band in CO(NH2)2. However, this latter in1374(10) 1397(6) 1609( 2 ) terpretation is not reasonable since the 1164 (1) 1709(3) 1667(1) cm.-I band is observed to be depolarized while tlie 1167(4) cm. band is polarized. The substitution 2021? 2902(2) 2906(5) of two ND2 groups for the two NH2 groups of urea 3018? (hsh) should eliminate the almost D a h symmetry of the -3400(bvs) -3390(bvs) heavy groups (0 and the two NH2) and should not result in the change of a band from polarized to depolarized. It must be concluded that the 1163 Discussion cm.-' band of CO(ND2)*does not correspond to The diffraction data of Ladell and PostI3 have the 1167 cm.-' band of CO(NH2)p. The formashown that in the crystalline state the C-N bond mide spectrum is so similar to that of urea that it has about as much double bond character as does is reasonable to assume that the 1115 em.-' bantl the C-0 bond. This double bond character has of HCOND2does not correspond to the 1093 cm.-I been attributed to the resonating forms band of HCONH2. This leaves the 913 c m - ' H ( C : :O):NIlz and H ( C : O - ) : :NH2+ band of HCONHz as corresponding to the 1093 The formamide molecule should then be planar cm-1 band of HCONH2. Additional data or normal to decide thc and have twelve fundamental vibrations. Using coordinate calculations are necessary relationship between 1221 and 1115 cm.-I o f the f r e q ~ e n c i e sof ' ~ liquid formamide, the more cer- HCOKD2 and 1600 ern.-' of HCONH2. tain assignments are given in Table 11. These Although the 1093 cm.-' band probably conassignments agree with those of Evans7 and Miya- sists mostly of planar rocking of the NH2 hydroza~va.~ gens, the 1.20 ratio between 1093 cm.-' and 9tR TABLE I1 cm.-l shows that other vibrations must be contributing. jvIBRATIOSAL L4SSIGXMES'13 The out-of-plane NH2 vibrations (wagging a i d IICONII1, HCONP, Class Form cm.-' cm. torsional) cannot be assigned with any reasonable certainty from our data. v,(SH?) 3330 2630 a' va(KH?) 3190 :I ' 2360 The HC1 Spectra.-The extreme broadening m t l a' Y(CH) 2889 2902 doubling of the 2900 c m - l band of formamide on a' VL( O C S ) 1680 1G70 dissolving in Concentrated HCl must not be intcr:1' S(CI1) 1390 1397 pretcd as meaning that the 11-atom on carbon has ;1' Vh(OCIN) 1x11 1.343 been greatly changed, since the 6(C-H) band a t 602 5117 ;I' 6(OCN) 1300 cm.-l does not become so diffuse and is not a" *(CJ-I) 1051 1048 doubled. A reasonable interpretation of these dlvergent effects can be obtained from the work of 'I'he remaining four vibrations belong to the Edsall and ScheinbergL5on methylamine and hyamide hydrogens. The work of Edsall and Schein- drazine. In CH3NH2 bands were observed at bergI5 on CH3ND2fixed 6(NH2) a t 1614 cm.-l in 3322 cm.-1 and 3382 cm.-', whereas in CH3NITL' CH3NH2and 1212 cm.-' in CH3ND2. A band is a weak diffuse band was observed near 2980 tin- l observed in the infrared and Raman of HCONH2 which was attributed to the NH3+ group. 111 a t 1600 ern.-' and this band disappears on deutera- +H3N-NH2 a broad band was found a t 2963 cin.-l. tion. A band appears a t 1221 cm.-l in the infra- Our data can be interpreted in the following way: red spectrum and one a t 1115 ern.-' in the infrared when formamide is dissolved in concentrated hyand Raman spectra of HCOND2. The 1221 ern.-' drochloric acid a proton is captured by the amido band gives a better wave number ratio (1.31 as nitrogen, forniing HCONHd+. The symmetrical compared with 1.41for the 1115 cm.-l band) and stretching frequency of the hydrogens in -NH3+ agrees more closely with the value observed by enters into Fermi resonance with the C-H stretchEdsall and Scheinberg,'j but was not observed in ing frequency producing a very diffuse doubleheaded band in the 2900-3030 cm.-' region. The (11) Intensities were obtained from microphotometer tracing of t h e ipcctra. proton capture must involve most of the foriiia( I ? ) Bvs. broad and very s t r o n x ; bsh, broad shoulder. T h e mide since the usual C-H stretching band could not Karnan hnnds in t h e 3100 cin.-l region in the aqueous solutions are be observed. This interpretation is supported b), chiefly due t o t h e water prcsent. T h e strong water hands cover u p the shift of the carbonyl stretching vibration t o :any hands d u e to other rnulecules ( ! 3 ) 1 l,arlel! an,] B. F'uwt Iiiglicr values (16% cni.-' in liquid IICONII,, TABLE
1
OBSERVEDRAMAS'II'AVII XUMBERS ( C M . - ~ )A N D INTENSI- appeared in the Raman.
(1.1) (1.5)
111t h e diictisG
