Infrared spectra of nitrogen containing compounds—I

Spectrochimica .kta, 1962,vol. 18,pp.1817to 1289. Pergamon Press Ltd. Printed inNorthern Ireland

Infrared spectra of nitrogen containing compounds-1 Benzamide* R. N. KNISELEY, V. A. FASSEL, E. L. FARQUHAR and L. S. GRAY? Institute for Atomic Research and Department of Chemistry Iowa State University, Ames, Iowa (Received

Abstract-A

1 March

1960; in revised form

study of the infrared spectra suggests that the frequencies of the amide-I spectra of benzamide, i.e. the amide-II band Other frequencies which may be useful for identified.

9 February

1962)

of benzamide, benzamide-lsN and benzamide-d2 and amide-II bands are inverted in the solid-state lies at a higher frequency than the amide-l band. characterizing the primary amide group are also

INTRODUCTION IT IS often difficult to treat the infrared spectra of nitrogen containing compounds in a systematic fashion. The introduction of a nitrogen atom into an organic compound usually involves extensive and often subtle changes in the electronic distribution of the parent molecule. As a consequence the normal concepts of isolated functional group frequencies may not apply to these compounds. Further complications arise from the tendency of many nitrogen containing compounds to undergo inter- or intramolecular association and to interact with solvents. These molecular interactions may produce pronounced frequency shifts which may cause difficulties in correlating the bands between the spectra run under different conditions. Isotopic substitution is a valuable tool in the identification of these characteristic frequencies. Selective deuteration combined with the use of l5N substitution provides a means of determining which absorption frequencies involve significant hydrogen Some preliminary data on 15N substitution has been reand nitrogen movement. ported previously [ 11. The present communication presents the results obtained for benzamide. EXPERIMENTAL

Benzamide-d, was prepared by a direct exchange of benzamide with D,O. Four exchanges with a 200 per cent excess of D,O were sufficient to provide almost The benzamide-d, was handled in a dry box complete conversion to benzamide-d,. in order to prevent back exchange with atmospheric water. BenzamideJ5N was prepared using the method of FONES and WHITE [2]. The source of l5N was (NH&SO, having an isotopic purity of greater than 95 atom per cent 15N. The infrared spectra were obtained with a Perkin-Elmer Model-112 double-pass prism spectrometer, a Perkin-Elmer Model- 13 double-beam ratio recording prism * Contribution No. 556. Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. t Present address: Armour Industrial Chemical Company, Chicago, Illinois. [l]

L. S. GRAY, V. A. FASSEL and R. N. KNISELEY, Spectrochim. Acta

[23 W. S. FONES and J. WHITE, J. Arch. Biochem. 20, 115 (1949). 1217

16, 514 (1960).

1218

R. N. KNISELEY, V. A. FA~SEL, E. L. FARQUHARand L. S. GRAY

spectrometer and a Beckman IR-7 prism-grating spectrometer. The prism instruments were used in the frequency range 5000 cm-r-420 cm-l with LiF, CaF,, NaCl and KBr prisms installed in the appropriate regions. These instruments were calibrated in accordance with standard procedures [3]. The IR-7 instrument was used over the region from 4000 cm-l to 650 cm-l ; the factory calibration for this instrument was “spot checked” against atmospheric HZ0 and CO, bands.

CRYSThL

NlJdOL

K Br PELLET

K Br PELLET LYoPHlLlZED

l7w-FREQUEZ? (cm-h Fig. 1.

The spectra of benzamide and its isotopic derivatives were recorded as CHCl, and Ccl, solutions, as melts at ~140”, as crystallized melts at room temperature and as KBr disks. It is relevant to note that the deuterium in the benzamide-d, exchanged rather readily with the residual water in the KBr. Satisfactory spectra could be obtained only if the KBr were “freeze-dried” and all sample preparations were conducted in a dry box to avoid further water pickup. Some of the bands in the solid state spectra also showed considerable variations [3] A. R. DO~NIE, M. C. MAGOON,T. PURCELLand B. CRAWFORDJr., J. Opt. Sot. Am. 43, 941 (1953).

Infrared spectra of nitrogen containing compounds-I

1219

in intensity, as illustrated by the bands at 1658 and 1624 cm-l in normal benzamide. Fig. 1 shows these two bands as recorded in different solid state samples. In the crystal and Nujol-mull spectra the 1658 cm-l band is considerably weaker than the 1624 cm-1 band. In the KBr pellet prepared by grinding the sample and KBr together, the two absorptions are about equal and both appear broader. In the pellet prepared by lyophilizing a mixture of KBr and benzamide, the intensity of the 1658 cm-l band exceeds the intensity of the 1624 cm-l band and again the bands are broad. Another marked intensity change occurred in the KBr pellet spectra of benzamide d,. Using benzamide-d, recrystallized from D,O the band at 1211 cm-l showed a relative intensity of approximately 1. However, when the same compound was recrystallized from a melt, the intensity of this band increased to approximately 4. The spectrum of the latter more closely approximated the pure-crystal spectrum. Table 1. Variation of average measured frkquencies and Av values for a typical set of data obtained for the 1625 cm-l band in five different KBr pellets of the 14N and 15N compounds Frequency (cm-l)

Ave.

Av (cm-l)

14NH,

15NH,

1624 1624 1626 1625 1625

1620 1623 1624 1623 1623

4 1 2 2 2

1623

2

1625 Mean deviation

=

kO.8 cm-l.

Because of partial polarization of radiation by the monochromator, the orientation of some of the crystals in the optical path also gave rise to intensity changes. In cases where this occurred, the intensities reported are the average of the two orientations. Since 15N substitution in benzamide involves frequency shifts of only 2-8 wavenumber, special care was taken in all of the frequency measurements. The following scheme was employed to accurately determine the l5N shifts. Two or three frequency measurements were made on each recorded spectrum, using different fiducial marks corresponding to known calibration points. The average of these frequencies was used to determine the 15N shift for each sequential pair of 14N and 15N spectra. To illustrate the precision of the frequency-shift determinations, Table 1 shows a typical series of average frequencies obtained from a sequential series of measurements on five different KBr pellets of the 14N and lsN compounds. All of the Av values in Tables 2 and 3 were obtained from similar measurements and in all cases they show a mean deviation of less than 0.9 cm-l. A spectrum of benzamide is shown in Fig. 2(a) and the frequencies of the bands in benzamide and benzamideJsN are listed in Table 4. A spectrum of benzamide-d,

1220

R. N. KNISELEY, V. A. FASSEL, E. L. FARQUHAR and L. S. GRAY

Table 2. Frequency ratios and shifts of amide-I-amide-II bands, using assignments of WECKHERLIN and L~~TKE and others Amide-I

Solution Melt Crystal KBr

Solution Melt Crystal KBr

1676 1660 1656 1658

1669 1650 1631 1630

14NH,

15NH,

1676 1660 1656 1658

1674 1658 1650 1653

Table 3. Frequency

band

1.005 1.006 I.015 1.017

Solution Melt Crystal KBr

Solution Crystal KBr

1194 1207 1211 1211

Av

14NH,

15NH,

Av

-2 -2 -6 -5

1587 1608 1626 1624

1579 1602 1624 1623

-8 -6 -2 -1

1669 1650 1631 1630

14NH,

15NHs

1676 1660 1626 1624

1674 1658 1624 1623

14Nd,

l”Nd 2 ____

1669 1631 1630

1667 1630 1629

Amide-II

1.005 1.006 0.997 0.996 Av -2 -2 -2 -1 Av -2 -1 -1

1.327 1.331 1.343 1.343

bands using inverted assigmnents

band

1676 1660 1626 1624

band

1587 1608 1626 1624

ratios and shifts of amide-I-alnide-II Amide-I

Solution Melt Crystal KBr

Amide-II

band

1587 1608 1656 1658

1194 1207 1211 1211

14NH,

15NH,

1587 1608 1656 1658

1579 1602 1650 1653

14Nd,

15Nd,

1194 1211 1211

1189 1205 1206

1.327 1.332 1.367 1.369 A&P -8 -6 -6 -5 Av -5 -6 -5

is shown in Fig. 2(b) and Table 5 is a tabulation of the frequencies observed for benzamide-cl, and benzamide-16Nd,. The intensities are estimates based on the peak transmittance of the bands, the strongest absorption in each spectrum having been arbitrarily assigned an intensity of 10. Table 6 summarizes the frequencies observed for the more important primary amide group vibrations together with the isotopic shifts observed for these bands.

Infrared spectra of nitrogen containing compounds-I

1221

I

I

DISCUSSION Prior investigations on the infrared spectra of primary amides have led to the identification of several bands which are useful for characterizing these compounds. The NH, stretching vibrations have been explored to the greatest extent, particularly with respect to the effects of intermolecular bonding on the frequencies of these bands [4-91. These studies indicate that very strong intermolecular hydrogen bonding occurs in the solid state, in agreement with the crystal structure as determined by diffraction studies [lo]. The shifts observed for these bands upon 15N and deuterium substitution are consistent with the conclusions reached in the earlier investigat,ions. Three other bands, commonly referred to as the amide-I, amide-II and amide-III bands, occur in the 1700-1300 cm-l region and are considered as charact’eristic of primary amides. The probable vibration modes which give rise t,o these absorptions [4] H. M. RANDALL, R. G. FOWLER, N. FUSON and J. R. DANGL, Infrared Determination of Organic Structures p. 10. Van Nostrand, New York (1949). [5] R. E. RICHARDS and H. W. THOMPSON, J. Chem. Sot.1248 (1947). [6] S. E. DARMON and G. B. B. M. SUTHERLAND, Nature 164, 440 (1949). [7] S. MIZUSHIMA, T. SHIMANOUCHI and M. TAUBOI, Nature 166,406 (1950). [S] L. J. BELLAMY, The Infrared Spectra of Complex Molecules p. 203. John Wiley, New York (1958). [9] R. N. JONES and C. SANDORFY Chemical Applications of Spectroscopy p. 247. Interscience, New York (1956). [lo] B. R. PENFOLD and J. C. B. WHITE, Acta Cryst. 12, 130 (1959).

10

4 5

1674

1604 1579

1580

1676

1605 1587

1582

1694

3540 3503 3420

14N

T

1692

10

3 sh 3

Cntensity

_T

* Saturated solution in 0.2-mm cells. t Saturated solution in 2.5-mm cells.

5

3 sh 4 <1 sh <1 3

3522 3503 3405

3532 3505 3415 3348 3300 3180 3020

_-

16N

‘4N

lnteusity

Soln. (CHCl,) *

1576

1608 1575

1602

1658

3183

3195

1660

3458 3323

3470 3335

16N

7

8

10

5

5 7

1616 1604 1656

1578

1618 1603 1650

1626

1578

1758 1717 1624

3083 3070 3034 1974 1954 1911 1890

3082 3070 3032 1974 1954 1911 1891 1806 1765

‘KN

3365 3303 3175

14N

8

sh sh 8

sh sh sh <1 1 <1 1 <1
sh 8

9

Intensity

Crystal

3370 3305 3178

T

Intensity

Melt (~140°C)

Table 4. Freauencies observed in undeuterated benzamide

1577

1618 1603 1653

1616 1603 1658 1577

1623

3078 3065 3030

3355 3292 3161

1624

3079 3064 3028

3367 3306 3173

‘JN

8

sh sh 10

8

ah sh sh

sh 8

8

Intensity

KBr pellet

1610 in n-butyl alcohol

1694 in CS,, 1668 in n-butyl alcohol

in CS,

Remarks

3538 3505 3419 I

T

--

1520 1510 1496 1449 1372 1302 1245 1185

<1 <1
CntensitJ

1181

1180

‘5N

1450 1355 1301

‘4N

1

1 6 1

intensity

soln. (Ccl&t

1449 1359 1301

T

* Saturated s6lution on 0.2-mm cells. t Saturated solution in 2.5~mm cells.

1520 1510 1495 1450 1375 1301 1247 1184

‘SN

Soln. (CHCl,) *

1183 1131 1103 1073 1026 1001

928 803 757 716 695

928 805 758 716 695

1493 1447 1376 1299

1558 1539 1493 1446 1380 1300 1243 1183 1135 1107 1073 1026 1001

‘4N

T

sh 3 3

sh


<1 <1 <1 5 8 3 <1 1 <1 tl sh 2 <1

Intensity

T

1450 1408 1301 1250 1185 1144 1123 1073 1027 1001 987 968 923 850 805 794 771 705 687 636

‘4N

Table 4 (Contd). Melt (-140”)

1249 1183 1141 1117 1073 1026 1001 987 968 924 847 806 796 770 703 687 633

1495 1449 1404

1SN

1 3 8 3 <1 <1 4 4 <1 2 2 <1 <1 2 <1 5 4 5 sh 5 4

Intensity

Crystal

T

<1 1 3 <1 3 <1 <1 <1 <1 2 3 2 6 6 4 2 3

925 849 808 789 769 706 686 633 621 526 925 849 810 791 771 705 685 636 620 529

<1 4 8 2

Intensity

1181 1140 1115 1073 1026 1001 987

1448 1398 1300

‘SN

1181 1142 1122 1073 1026 1001 987

1495 1449 1402 1298

‘4N

KBr pellet Remarks

767I

789

in CS,

1358 in CS,

T

2745 2675 2633 2572 2488 1667 1620 1604 1580

1506 1496 1451 1425 1378

2640 2575 2492 1669 1621 1605 1578

1506 1495 1451 1433 1386


4

1

2 <1 3 10 sh 2 4

3




Intensity

* Saturat,ed solution in 0.2-mm

3020

3470 3455

‘5N

3020

‘4N

Soln. (CHCl,) *

cells.

1194 1180

1689

l*N

1189 1180

1685

IvT

Soln. (CCl,)t

2 1

925 802

solution in 2.5-mm

sh 2

Sh

2 3

1073 1027

711 692 666

2

6 2

1207

5

1396 1297


sh 7

3 4 4 10

Intensity

1497 1449

1605 1578

2587 2420 2395 1650

3062 3035

14N

Melt (-140)

observed in deuterated

t Saturated

6 1

10

Intensity

Table 5. Frequencies

cells.

717 692 667

1420 1299 1235 1211 1185 1105 1076 1027 1001 938 921 802

2530 2450 2372 1631 1619 1603 1576 1560 1522 1500 1450

3370 3084 3070 3030

3609 3582

‘*N

716 693 666

801

1412 1298 1232 1205 1185 1105 1076 1027 1001 934

1517 1500 1451

2525 2443 2366 1630 1619 1603 1576

l6N

Crystal

benzemide

8 3 <1 4 1 <1 3
8 sh 8 10 sh sh 7 sh <1 <1 5

:

Intensity

798 738 715 688 606 557 478

606 559 480

1206 1179 1105 1075 1025 1001 931

1406 1296

1517 1497 1450

2521 2445 2358 1629 1619 1603 1575

16N

798 738 715 688

1211 1179 1105 1075 1025 1001 936

1416 1296

1497 1451

2528 2446 2361 1630 1618 1602 1576

3063 3033

l4N

KBr pellet

<1


2 sh 7 4

8

4 5

4 2

8 3

sh sh 8

7 sh

Intensity

1674 1579 1372

-

1669 1194’ 1386

2640 2575 2492

1-4Nd,

._ -.._

1667 1189” 1378

2633 2572 2488

Wd,

* Measured in WI, solution.

1676 1587 1375

14NE4 ’ 5NH, _.-__ -_ 3532 3522 3505 3503 3415 3405

-

3458 3183 1658 1602 1376 1131

1103

1107 -

-

925

2587 2395 1650 1207 1396 1073

1

1123 636

3370 3178 1626 1656 1408 1144

_L

I14Nd,

L.

--

1-5NH,

3470 3195 1660 1608 1380 1135

1“NH, .-

-

1117 633

3365 3175 1624 1650 1404 1141

938

2530 2372 1631 1211 1420 1076

WdZ

Frequencies (cm-‘)

Frequencies (cm-l)

Frequencies (cm-r)

___.

Crystal

Melt (~140’)

CHCl, solution

-

15Nd,

L

3367 3173 1624 1658 1402 1142

1122 636

934 i

KBr pellet

2528 2361 1630 1211 1416 1075

936 559

3355 3161 1623 1653 1398 1140

1115 633

Frequencies (cm-l) __I_~ 15NH, 14NH, r4Nd, ~ -____ -.-

2525 2366 1630 1205 1412 1076

__

-

931 557

I

15Nd2

2521 2358 I 1629 1206 1406 1075

Table 6. Summary of frequencies and isotopic shifts for selected bands in benzamide

-

-_ NH, stretching (free) NH, stretching (bonded) amide-I band amide-II band amide-III band X-sensitive A, ring vibration (“!I”) NH, rook NH, wag (?)

I 1%

E g*

% S p 3 D g

1226

R. N. KNISELEY, V. A. FASSEL, E. L. FARQUHARand L. S. GRAY

have been discussed by several authors [4,5, 8, 9, 11-131. In general the amide-I band is assigned as the C=O stretching mode, the amide-II as the NH, deformation and the amide-III as the C-N stretching mode. However, in recent years there has been a tendency to treat the O=C-NH, group as a single vibrational unit. On this ba.sis, these three bands have been assigned as strongly coupled vibrations involving O=C-N stretching and NH, deformation motions. After this manuscript was originally submitted for publication a report of a similar study by WECKHERLIN and L~TTKE appeared in the literature [la],as well as a paper by PINCHAS et al. [15] on the study of the infrared spectrum of benzamide160. Our data and assignments for this very important 1700-1300 cm-l region are somewhat different from those reported by these authors. Since three characteristic frequencies of the primary amide group occur in this frequency range, it is of particular interesb to examine this region in detail. Since the solution spectra are somewhat simpler to interpret, it is well to discuss these first, considering mainly the information which can be obtained from isotope substitution. The top part of Fig. 3 shows a portion of the spectrum of benzamide and its deuterium isomer as observed in CHCI, solution. The prominent band at 1676 cm-l can easily be identified as the amide-I band. As shown by the data in Table 6, the frequency of this band is essentially insensitive to either 15N or deuterium substitution. The absence of significant isotopic shifts is in agreement with assignments which attribute the amide-I band to a C=O stretching mode. This is also in agreement with the 180 data obtained by PINCHAS et al. [15]. The amide-II band is normally observed in the 1570-1650 cm-l region. In CHCI, solution there are three bands of moderate intensity (1582, 1587 and 1605 cm-l) which lie within this frequency range (Fig. 3). The data in Tables 4 and 5 show that only the 1587 cm-l absorption is sensitive to 15N or deuterium substitution and it is therefore assigned as the amide-II band. PINCHAS et al. [15] apparently did not resolve the 1587 and 1582 cm-l bands because only a single frequency at 1584 cm-l was listed as the amide-II band. They reported a 10 cm-l shift in the 1584 cm-l frequency upon la0 substitution. However, the significance of this shift is difficult to assess since two bands of totally different origin are involved, the amide-II band and the B1vc_-C (‘7”) mode [ 161 of the phenyl ring. Assuming that the amide-II band is primarily an -NH, scissor vibration, the expected frequency in the Nd, analogs is in the 1200 cm-l region. Although this frequency range is blanked by the CHCl, solvent, the spectrum of benzamide-d, in CHCI, shows an indication of a band near 1215 cm-l and a new band of moderate intensity appears at 1194 cm-l in the Ccl, spectrum of the deuterated derivative. The 1194 cm-1 band also shifts to a lower frequency in benzamide-15Nd, lending [ll] A. M. BUSWELL, W. R. RODEBUSHand M. F. ROY, J. Am. Chem. Sot. 60, 2444 (1938). [12] A. M. BUSWELL and R. C. GORE,J. Phys. Chem. 46, 575 (1942). [13] H. W. THOMPSON,D. L. NICHOLSONand L. N. SHORT,DisczcssionsE”aradaySot. 9, (1950). [ 141S. WECKHERLINand W. L~~TTKE,2. Electrochem. 84, 1228 (1960). [15] S. PINCHAS,D. SAMUELand M. WEISS-BRODAY,J. Chem. Sot. 1688 (1961). [16] R. R. RANDLE and D. H. WHIFFEN, Molecular Spectroscopy p. 111. Institute of Petroleum, London (1955).

Infrared spectra of nitrogen containing compounds-I

1227

further support to the assignment of this band to the amide-11 band in the cleuteratecl derivative. Although these observations are in reasonable accord with an -NH, scissor vibration, the data discussed below indicate some degree of coupling with the amide-III band. In CHCI, solutions the amide-III band is found at 1375 cm-l in normal benzamicle. This band shifts only 3 cm-l lower upon 15N substitution and shifts 11 cm-l higher in frequency upon deuteration. In themselves these shifts do not appear to be

Fig. 3.

consistent with the usual assignment of the amide-III band to a C-N stretching mode. However, in the deuteratecl derivative, the substitution of 15N causes an 8 cm-l decrease in frequency, considerably greater than in the undeuterated compound. Referring to the solid state data, it is seen that the band near 1142 cm-l, which probably corresponds to the X-sensitive A, (“q”) mode [ 161 of the phenyl ring, also shows a definite sensitivity to cleuteration, as well as a slight sensitivity to 15N substitution. The -NH, rock, near 1122 cm-l, is also sensitive to 15N and deuterium

1228

R. N. KNISELEY, V. A. FASSEL, E. L. FARQUHAR and L. S. GRAY

substitution. Thus it appears reasonable to assume that there is coupling between the in-plane bending modes of the NH, group and the skeletal stretching modes of the amide group. In the molten state the amide-I, -11 and -111 bands are definitely displaced from the frequencies observed in the solution spectra. Measurement of the isotopic shifts identifies the 1160 cm-l absorption as the amide-I band, 1608 cm-l as the amide-II frequency and 1380 cm-l as the amide-III band. The shifts observed for these bands are almost identical to those observed for the solution spectra although the broadness of the bands lowers the accuracy of the measurements slightly. The frequency assignments for these three bands agree with WECKHERLIN and L~~TTKE [14]. As compared with the solution spectra, the solid-state spectra of primary amides show a pronounced decrease in the frequency of the amide-I band and a concurrent increase in the amide-II frequency [8, 91. These frequency shifts are normally attributed to the formation of strong intermolecular hydrogen bonds, although CANNON [17] has shown that dipole interactions provide a more satisfactory explanation for the small shift in the NH, stretching frequencies relative to the large change in the C=O stretching frequency. As a result of these frequency shifts the solid state spectrum of benzamide has two strong bands in the 1620-1660 cm-l region as shown in the upper portion of Fig. 2. These two bands can logically be associated with the amide-I and amide-II bands. If the assignments of WECKHERLIN and L~TTKE [ 141 are used for the amide-I and -11 bands, the isotopic frequency ratios shown in Table 2 are obtained. Although the frequency ratios for the hydrogen-deuterium compounds are not inconsistent with this assignment, the data obtained from 15N substitution produce a contradictory pattern of frequency shifts. These data are summarized in the lower portion of Table 2. As noted previously in this discussion, the amide-I band in the solution and melt spectra is not significantly sensitive to 15N substitution, whereas the amideII band shows an appreciable i6N isotopic shift. In the crystal and KBr pellet spectra the higher-frequency bands (1656 and 1658 cm-l respectively) show an 15N isotopic shift comparable in magnitude to that measured for the amide-II band in the solution and melt syjectra. Conversely, the lower-frequency bands (1624 and 1626 cm-i) in the solid-state spectra do not exhibit any significant l5N shift. A more consistent pattern of 16N shifts is obtained if the amide-I and -11 frequencies are inverted in the solid-state spectrum, i.e. the amide-II band lying at the higher frequency.* Table 3 presents the data on the basis of this inverted assignment. Other data may be cited which lend credence to the inverted assignment. In the solid-state spectrum of the l4Nd, compound there is only a single band at 1631 cm-l which is definitely the amide-I vibration. The data in Table 3 show that the substitution of lsN into this compound causes an insignificant shift of the band. Thus, the 1631 cm-l solid-state band in benzamide-d,, the 1626 cm-l solid-state band in * RANDELL et al. [4] implied the possibility of such a frequency inversion in their statement that “a band of less intensity than the carbonyl bond. . . . . occurs in the immediate vicinity of the carbonyl-group bond, is frequently poorly resolved from it, and may be found on either side of it.” This inversion was not illustrated in the spectra of amides which they reproduced. [17] C. G. CANNON, Microchim.

Acta

555 (1955).

Infrared spectra of nitrogen containing compounds-I

1229

normal benzamide and the band near 1670 cm-l in the solution spectrum of benzamide and the band near 1670 cm-l in the solution spectrum of benzamide all show identical behavior toward 15N substitution. On the other hand, the amide-II band in the 14Nd, derivative, which now lies near 1200 cm-l, exhibits a 16N shift similar to the 1656 cm-l band in the solid-state spectrum. The behavior of the 3305 cm-l shoulder on the asymmetric NH, stretching band in the solid-state spectrum is also consistent with an inverted assignment WECKHERLIN and L~TTKE [14] assigned the shoulder to either the first overtone of the amide-II band or to a combination of the amide-I and amide-II frequencies. BADGER and PULLIN [18] concurred with the former of these two assignments. The sensitivity shown by this band to both 15N and deuterium substitution confirms its assignment as the first overtone of the amide-II absorption. In view of this, the assignment of the 1656 cm-l absorption as the amide-II band is more consistent (2 x II = 3312 cm-l) than is the assignment of the 1626 cm-l absorption as the amide-II band (2 x II = 3252 cm-l). Although the data discussed above lend support to the inverted assignment of the amide-I and -11 bands in the solid-state spectra, they also present a serious inconsistency, namely, the 0.996 frequency ratio observed for the amide-I band for the normal and deuterated derivative. Although we cannot account for the occurrence of this reverse shift, it is pertinent to mention that an expansion of intermolecular hydrogen bonds has been observed in strongly hydrogen bonded crystals [ 191. An extensive study of N-H * * * 0 bonds with respect to this expansion has not been made nor are we aware of any studies on the effect of this bond expansion on infrared frequencies. Although our assignments in Tables 4, 5 and 6 are made on the basis of the amide-I amide-II frequency inversion, we should emphasize that the data are susceptible to different interpretation. We should also stress that the inverted assignments do not imply that the origins of the amide-I and -11 bands in the solution state are necessarily the same as in the solid state. Our data simply indicate that the 1656 cm-l frequency in the solid state is more closely related to the amide-II solution band than to the amide-I solution band. Similarly, the 1626 cm-i frequency in the solid state is more closely related to the amide-I solution band. Further studies on the behavior of these frequencies in other primary amides may permit a more precise description. The behavior of the amide-III band in the solid-state spectra parallels its behavior in the solution spectra. The data are also consistent with the concept that the individual group vibrations within the C-CO-NH, system are strongly coupled as discussed above. Several other bands in the KBr pellet spectra of benzamide show a definite Since the correlation of some of the bands of sensitivity to isotope substitution. benzamide and benzamide-d, involves considerable subjective judgment, definitive assignments have not been made. However, several bands in this region may hold promise as characteristic frequencies for primary amides. Thus, the band near 1122 cm-l has previously been assigned to the NH, rocking mode [ 141. The correlation of the 1122 cm-l band with the 936 cm-l frequency in benzamide-d, is in harmony with this assignment although there is an intensity discrepancy between these two

1230

R. N.

KNISELEY,V. A. FASSEL,E. L. FARQIJHAR and L. S. GRAY

bands. As mentioned above, the 1142 cm-l band is apparently the X-sensitive A, (“q”) mode [16] of the phenyl ring and the isotopic shifts observed indicate that this band is coupled with some of the amide-group vibrations. The band at 636 cm-l is both 16N and deuterium sensitive and is assigned as the NH, wagging mode in agreement with WECKHERLIN and L~TTKE. Other low-frequency bands also show very definite isotopic shifts. Further work on ring substituted benzamides should provide valuable data concerning the nature of these vibrations. Acknowledgements-The authors wish to express their gratitude to Dr. F. H. SPEDDING and J. POWELL for supplying the 16N used in this investigation.

[18]R. M. BADGER and A. D. E. PTJTXN, J. Chem. Phys. 22, 1142 (1954). [19] K. J. GALLAGINER,Hydrogen

Bonding

p. 45.

Pergamon, London (1959).

Dr.