RAX4N SPECTRA O F ANIXO ACIDS AND RELATED COMPOUNDS. I1 GUANIDINE AND UREA DERIVATIVES' JOHN T. EDSALL The Department of Physical Chemistry, Harvard Medical School, Boston, Massachzisetts Received September 28, 1936
The structure of guanidine and urea and their derivatives has long been a matter of debate among chemists, in spite of the small size and apparent simplicity of these molecules. The study of Ranian spectra should be of aid in recognizing the nature of the chemical bonds involved and in evaluating their force constants, also in throwing some light on the type of symmetry present in the molecules. For this purpose, the Raman spectra of guanidine and methylguanidine hydrochlorides, methylurea, thiourea and thioacetamide have been determined; that of urea is already known (2, 5 ) . Qualitative polarization measurements have also been made for the Raman lines of guanidine hydrochloride. Even when not quantitative, such measurements are often of value in assigning frequencies to modes of molecular vibration. Taken in conjunction with observations from other sources, especially x-ray diffraction data, the observed Raman spectra appear to permit the drawing of some definite conclusions regarding the structure of the niolecules. Further consideration also indicates that the formate ion, whose spectrum has been preriously determined (2), bears a close resemblance in niolecular symmetry to the other molecules and ions here considered. Tentative analysis of its Raman spectrum is undertaken with a result which may be of more general significance in throwing light on the structure of the ionized carboxyl group in other acids. EXPERIMEXTAL METHODS
The experimental technique employed for exciting the Raman spectra was essentially the same previously described ( 2 ) , being modeled closely on that dereloped by R . IT. ITood. Exposures were always taken in filtered light. To isolate the 4037 mercury line, a dilute solution of sodium nitrite Presented a t t h r Symposium on Molecular Structure, held a t Princeton r n i v e r sity, Princeton, New Jersey, December 31,1936 t o January 2,1937, under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society. 133
134
J O H N T. EDSALL
(about 0.02 saturated in a layer 30 nini. thick) n-a. uhed t o remove all lines below 4000 A.U., and Corning red-purple ultra glass (2 mm thick) t o remove the 4358 line and all line.: of longer lvave length. To transmit 4358 A.U., removing 404T and all shorter wave lengths, a dilute solution of p-nitrotoluene in alcohol was employed (1). Expohures were taken with both filters for every substance examined except thiourea and thioacetamide. In aqueoui .elution t h e v substances decompose, u n d ~ rthe influence of light, giving a very fine cloudy precipitate (sulfur ?), which produces such iiitense continuous background as t o mask the Raman lines. These solutions were, therefore, studied using T ery short exposures at very high light intensities, using either the p-nitrotoluene solution or dilute sodium nitrite (without the purple glass) as a filter. The Raman tube was watched carefully and, as soon as the cloudy precipitate began to form, the exposure was stopped (generally after about ten minutes). Under these circumstances a fairly satisfactory spectrum of thiourea was obtained, but the spectrum found for thioacetamide is certainly incomplete. Further details concerning the substances studied will be found in table 2, which gives a coiiiplete list of the spectra obtained. DISCUSSIOS
The observed spectra, including that of urea, which has been previously studied, are listed in table 2 and shown graphically in figure 1. The general similarity in the spectrum of urea and of the guanidoniuni ion is ob^ ious and striking, as might indeed have been expected from their general structural likeness. The close similarity between methylurea and the methylguanidonium ion is also immediately apparent. Thiourea shows a spectrum closely analogous to that of urea, but with most of the frequencies displaced to lower values. The incomplete spectrum of thioacetamide indicates the close relationship of this substance to thiourea. The general character of the spectra, taken in conjunction with what is knonn of the chemistry of these substances, indicates that these molecules all belong to a closely related group. I n the more detailed discushion of these spectra which follows, t\i o considerations are important. Both experiment and theory have shon-n ( 3 ) that oiily compounds containing covalent bond. give riqe to Raman frequencies, at any rate in the range with which we are here coiicernd ( > 300 cm.-l). Thus the Raman spectrum of guanidine hydrochloride is the spectruiii of the guanidonium ion, +C(SH2)3. I\Ioreover, in a first approximation, the NH2 group may be trcatecl a. a sirigle vibrating unit, urea and the guaiiidonium ion being conaidered for purposes of calculation as tetratomic molecules. Thc NH2 group, like the CH2 group, may of courhe be expected to give riw to characteristic internal valence and deformation frequcncie., which muqt hc allon-rd for in the analysiz of the observed spectra.
STRUCTURE O F G U A S I D I N E AND UREA DERIVATIVES
135
The most striking feature in thevery similar spectra of urea [O=C(?I"z)2] and the guanidonium ion [+C(KH2)J is the intense line which lies in both cases almost exactly at 1000 cin.-l It should be noted that the oxygen atom in urea has virtually the same mass as the KH2 groups, so that if the strength of the bonds is similar, the type of symmetry in the two compounds is not very unlike. Closer examination shows, however, that the spectrum of the guanidonium ion is distinctly simpler than that of urea, suggesting a higher type of niolecular symmetry in the former. Thus the line at 1170 cm.-l in urea has no counterpart in the spectrum of the guanidoiiiuni ion, and the one fairly strong line at 533 in the guanidonium ion is replaced b y two weaker lines a t 521 and 584 in urea. This may well represent the splitting of a doubly degenerate frequency due to a reduction of the symmetry of the molecule. Such a distinction between the two com0 (1.1 Nos-
400
I
-
121 co3-
1200
800
\
1600
cm-'
'I
I'IG. 1. Raman spectra of (1) the nitrate ion, (2) the carbonate ion, (3) the guanidoniuni ion, (4) urea, ( 5 ) thiourea, (6) methylurea, ( 7 ) the inethylguanidonium ion. T h e height of the Raman lines indicates roughly their relative intensities. A broad line is iiidicated by broadening a t the base. The lines above 1600 em.-' may be due, a t least in part, t o the water used as solvent.
poiuids should not be expected from the classical formulas, according to which urea was generally written as shown in formula I, and the guanidoniuni ion as shown in formula 11.
I Treating the KH2 groups as unit niasseq, and assuming that all the bonds lie in one plane, each of these structures should possess a twofold symmetry axis (symmetry Czt,). This indeed appears t o harmonize with the observed spectrum in the c a v of urea (although the classical formula probably re-
136
JOHS T. EDSALL
quires a revised interpretation). For the guanidoiiium ion, however, it has been shown on other grounds that the classical forniula is in need of revision. In formula I1 a double bond is shown a5 joining the carbon to nitrogen atoni 1. If i t n-ere attached instead, however, to either atom 2 or 3, the three resulting structures would be completely equivalent. Under these circumstances, as pointed out by Pauling, Brockway, and Beach (8), complete resonance between all these possible structures should occur, the resulting ion possessing trigonal symmetry (syniinetry the carbon and the nitrogen atoms being all coplanar: 2"
I
C
/+\
NH2
H2N
I11 This is exactly analogous t o the structures already demonstrated (9) for the carbonate and nitrate ions. The three C-K bonds are all equivalent, and in strength probably approach more nearly to double than to ordinary single bonds. The x-ray diffraction nieasurementi of Theilacker (10) on the guanidoniuni halide. are entirely iii harmony with this picture, the results indicating the complete equivalence of the three C--S bonds. The lorn carbon-nitrogen distance (about 1.18 A.U.) indicates that the bond is a very strong one. A molecule showing this type of symmetry -hould give rise to three Ramaii frequencies (9, 12) : one totally symmetric and polarized, vl, and two degenerate frequencies which are depolarized, v 2 aiid v 3 ( p = 6/7). (A fourth, asymmetric frequency is Raman-inactive, although present in infra-red absorption.) Polarization measurement. >how the intense line at 1008 em.-' to be highly polarized, while those at 533, 1566, and 1662 are almost completely depolarized. VI is therefore 1008 cm.-', while 533 certainly corresponds to one of the degenerate frequencies ( Q ) . v 3 may be represented either by the very m-eak line a t 1480, or by one of the somev-hat stronger lines at 1566 or 1662. One of these frequencies probably represents a deforniational vibration of the NH2 group, and that a t 1662 may be partly due t o the weak water band preieiit in thi. range. Any definitive assignment of these three frcqueiicies iq imposiible on the basis of thc present data. The force constants of the C--N bonds can, however, be approxiniatc.ly calculated if the guaiiidoniuni ion he treated as a valence force system (6). On this assumption, the binding constant, f, i. uniquely determined in terms of the frequency v 1 and the mass of the KHZ group, and is found to be 9.54 x lo5 dynes em.-'. Using this value and the observed value of v~ (533 cm.-l), we may then ,olw for d , the deformation constant of the C-K bond, and a140 calculate t h e third frequency, v3. Comparable cal-
137
STRUCTURE O F GUAKIDINE rlXD UREA DERIVATIVES
culations may also be made for the KOs- and COS-- ions, and the results are summarized in table 1. It is apparent that the calculated values of v 3 for the nitrate and carbonate ions are grossly in error. This discrepancy indicates the inadequacy of the assumptions involved in a valence force system to account for the behavior of a molecule of this type. It is therefore iiiipossible to use these calculations to decide Tyhich of the observed frequencies corresponds t o v 3 for the guaiiidoniuiii ion. It is perhaps significant, however, that the caJculated f values are much closer to those generally typical of double Iionds (10 to 12 X lo5) than to those of single bonds (4 to 5 X 105),-a result entirely in harmony with the expectations from Pauliiig's theory. Urea unquestionably possesses a lower degree of syninietry than the guanidonium ion. This coiiclusion follon s both from the Rainan spect r i m and from the x-ray diffraction measurements of Wyckoff and Corey (14),which sho\v the carboii-oxygen distance to be inarkeclly lower than the carbon-nitrogen diqtance. As would have been inferred froin the TABLE 1 Frequenczes and force consfants of the carbonate, nitrate, and guanidoniurn ions IOX
COS-x03 -
1065 1050
C(NH2)
1008
1
-1
j
714 720
1
533
1438 1360
j
1985 1870
10.65 X 10; 10.36 X l o 5
9.54
x
105
1.115 1.16
x x
105 105
0.578 x 105
classical formula, the syiniiietry is undoubtedly CZs; the niolecule should give risc to six fuiidanieiital frequencies, all present in the Ranian spectrum, three polarized and three depolarized (12). An attempt to analyze the spectrum of urea on this basis has already been made by Kohlrausch and Pongratz ( 5 ) , who have pointed out the inadequacy of the assumptions involved in a valence force treatment as applied to a system of this sort. I n their treatment, they assume (as would follow froni the classical formula) that the C--?J bond. have thestreiigth of singlebonds only. According to Pauling, Brockn-ay, and Beach (8)' however, resonance betxyeen the C=O and the two C--S bonds is nearly complete; hence the C-N bonds are probably closer to double than to true single bonds in strength. On no assumption, however, does the oversimplified valence force picture givc anything approaching a quantitative description of the facts. Th7ourea
T h e spectruin of this substance is of exactly the same type as that of urea. The powerful line a t 7 3 2 corresponds to the syninietrical vibration
138
JOHX T. EDSALL
at 1000 in urea; and the two weaker lines at 418 and 487 in thiourea appear to be the exact analogues of those a t 521 and 584 in urea. Not only is the sulfur atom more massive than oxygen, but the carbon to sulfur distance in thiourea, 1.64 A.U., (13) is much greater than the carbon to oxygen distance in urea. Furthermore, the force constant of the C=S bond is probably much less than that for the C=0 bond in urea, since its value for C-S in methyl mercaptan is only about three-fifths of that for C-0 in methyl alcohol (4). As with urea, however, no quantitative description b y a valence force system is possible. The very similar molecule, thioacetamide, in which the symmetry has been destroyed by replacing one NH2 b y a CH, group, shows as its principal vibration a frequency (T14 cni.-l) very close to that of thiourea.
X e t h y l urea and the rnet~i~lqua~i~~oniii?n ion (see .figure 1 ) These two substances possess Raman spectra extraordinarily alike, indicating that the pattern of molecular structure in the two is essentially identical. Evidently the difference between the C=0 bond in the one compound and the C-NHz bond in the other is very slight as compared with the disturbance of symmetry introduced by the presence of the methyl group in both compounds.
The formate ion Since there is n o obvious basis for distinguishing between the two oxygens in the ionized carboxyl group, resonance between them is probably coniplete, and the two c-0 bond. and c-0 distances should be identical. On this assumption, the formate ion,
H c'
/ \
0
0
IT; possesses a twofold axis of symmetry (symmetry group Czu) and is thui closely analogous to urea or to (unpolymerized) formaldehyde. The Raman spectrum of the ion has been previously determined ( 2 ) , and consists of the following lines (in cni.-l): 772 ( l ) ,10T2 (l), 13Z1 (6),1386 (1)) 1611 (0), 2123 ( l b ) , 2734 (2b), 2823 (Bb). The figures in parentheses indicate roughly the relative intensities of the line
