Normal Vibrations of N, N-Dimethylformamide and N, N

NORMAL VlBRATIONS OF N, N-DIMETHYLFORMA- MIDE AND N, N-DIMETHYLACETAMIDE

BY V. VENKATA CHALAPATHI AND K. VENKATA RAMIAH (Department of Physics, College of Science, Osmania Univer$ity, l-Iyderabad-7) Roceived Fr 21, 1968 (Communicated by Dr. N. A. Narasimham, F.A.SC.)

ABSTRACT

Infla.red and Raman frequencies of N, N-dimethylformamide and N,N-dimethylacetamide as recorded by the authors, in the region 3100 cm.-~ to 250 cm.-1 are given. The normal co-ordinate treatment of these molecules has been carried out using general quadratie foree field and the potential energy distribution of the various modes of vibrations ]lave been calculated to study the nature of absorption frequencies arising out of the in-plane vibrations. The assignments made by the authors in the region 3100 cm.-~ to 500 cm.-~ arecompared with those of Lumley Jones who assigned the frequencies on the basis of band contour studies. These calculations have enabled the authors also to assign the frequencies in the region 500 cm.-1 to 250 cm.-1 to the various bending modes of vibrations. The C--N stretching frequency in tertiary amides is considerably different from that in primary and secondary amides.

I. INTRODUCTION

SPECTP.O$COPIC studies of primary and secondary amides and the normal co-ordinate treatment of these molecules received considerable attention in recent times. Several authors x-n llave studied the infrared spectla of primary amides like foimamide, and acetamide and secondary amldes like l~-methyl- formamide and ~-methy and of their deuterated species in various states of aggregation to investigate the nature of intermdecular associations in these molecules and Miyazawa etal., Suzuki, Puranik and Lalitha Sir- deshmukh and the authors 5-7, 9-11 subjected these molecules to normal co-ordinate treatment in order to understand the nature of the absoIption frequencies arising out of different modes of vibrations. The main result that emerges out of these studies is that in primary amides, the amide I, the amide II and the amide III bands aro essentially due to v (C----O), (NH~) and v(C--N) vibrations respectively. On the other hand, in secondary Al 109 110 V. VENKATA CHALAPA'IHI AND K. VENKATA RAMIAH amides, while the amide I band is essentiMly a C--O stretch, thc amide II and the amidc III bands have been shown to arise out of the combined contribution of 3(NH) and u(C--N} vibrations. These results have bcen broadly supported from tke spectra of N-methylacetamide and acetanilide obtained by Bradbury and Elliofl ~ and Soichi Hayashi 13 who used polarised infrared radiation in tl'.ese studics. Lumley Jones 1~ assigned the vibrational flequencies of some secondary and tertiary amides in the region 3500 cm.-~ to 500 cm.-I on the basis of band contour studies of infrared absorption bands and Katon etal? 5 recorded the spectra of these compounds in the iegion 700 cm.-~ to 250 cm.-1 There is no earlier attempt to investigate the nature of the absorption frequencies arising out of the in-plane vibrations in tertiary amides on the basis of nolmal co-ordinate treatment. The authors have therefore chosen two simplest molecules of the tertiary amides--N, N-dimcthu and N, N-dimethylacetamide and subjected thcm to normal co-ordinate treatment by using the general quadratic force field.

II. EXPERIMENTAL AND RESULTS

The infrared spectra of N, N-dimethylformamide and N, N-dimethylacetamide in the region 3100 cm.-1 to 700 cm.-1 have been recorded using Perkin-Elmer Model 21 infrared spectrophotometer with NaC1 optics. The spectra of the liquids were recorded using microfilms of unknown thickness and of solutions in CCI~ with matched cells of 0.1 mm. thickness with NaCI windows. The spectra of the pure amides were also recorded in the region 700 cm.-~ to 400 cm.-~ with Perkin-Elmer Model 337 grating spectrophotometer, and in the region of 4C0 cm.-x to 250 cm.-~ with Pelkin- Elmer Model 521 grating spectrophotometer. The Raman spectra of these substances were recorded with Fuess glass spectrograph and Hilger's Raman source unit and A 4358 was used as the exciting radiation. The infrared and Raman fiequr as recorded by the authors and the assignments are given in Tablcs I and II and the corresponding infrared and Raman spectra are shown in Figs. 1, 2, 3 and 4. The infrared bands at 1256 cm.-1 and 1259 cm- 1 in the two amides ate strong whereas the ba~.ds at 865 cm.-1 and 738 cm.-~ are weak. But in Raman spectra, the frequencies at 866 cm.-~ and 743 cm.-~ ate the strongest lines while the other two frequencies at 1230 cm.-~ and 1259 cm.-1 are reIatively less intense. Similarly the Raman line at 960 cm.-1 which is assigned to v (C--CHs) vibration in N, N-dimethylacetamide is fairly strong whereas the corresponding band in infrared is wcak, Vibrations of N, N-D imethylformamide & N, N-Dimethyiacetamide 111

TABLE I

Infrared and Raman spectra of N, N-dimethylformamide (Frequenciesin cm.-1)

Infrared Raman

Amide in Assignment Pure amide solution Pure amide ofCC14

3000 (w) .. 3006 (2) v,, (CH3) N 2950 (m) 2933 2945 (5) v, (CH3) N 2875 (ras) 2870 2873 (5) v (C--H)

o. .. 2823 (1) 1664+1160 1685 (vs) 1685 1664 (4) v (C=O) 1502 (m) 1499 1442 (6) v (C--N)

1443 (m) 1435 ... (CH3) N 1404 (na) 1404 1407 (6) (CHa) N

1393 (s) 1381 .. ~ (C--H) 1256 (ras) 1256 1230 (3) v,, (N--CHa) 1149 (vw) 1149 1160 (2 b) ~o (CH3) N 1091 (vs) 1087 1094 (4 b) 9, (CHs) N 1064 (w) 1064 .. ~ (C--H) _1. 865 (w) 865 866 (6) v, (N--CHs) 662 (s) .. 667 (6) (O =C--N) 400 (w) .. 405 (4) (CHa--NmCHa) 355 (na) .. 354 (4) ~" (C--N) 320 (w) .. 316 (3) 9' (CHa--N--CH3) 112 V. VENKATA CItALAPATI-II AND K. VENKATA RAMIAH

TABLE I1 lnfrared and ~aman spectra of N, N-dimethylacetamide (Fr162 s in cm.-t)

Infrared Raman

Amide in Assignment Pure amide solution Pure amide of CCI~

3003 (m) 3020 3050 (3) v~ (CH3) N

2941 (ms) 2959 2929 (6) v, (CHa) N 2870 (w.sh) .. 2868 (4) yo, (CHa) C

2820 (w.sh) .. 2827 (7) v, (CHa) C 1653 (vs) 1657 1646 (6) v (C =O)

1494 (m) 1494 1452 (5) v (C--N) 1440 (m) 1440 .. ~~ (CHa) C, N

1395 (5) 1395 1412 (5) ~o (CHa) N

1351 (m) 1351 1359 (2) ~, (CHa) C

1259 (s) 1263 1259 (3) v~, (N--CHa) 1183 (s) 1185 1182 (3) oJ (CHe) N 1054 (m) 1056 .. ~, (CHa) C

1029 (m) 1029 .. r (CHa) N 1013 (s) 1013 1016 (2) r (CHa) C

957 (w) 957 960 (4) v (C--CHa) 738 (w) .. 743 (7) yo (N--CHa) 593 (s) .. 591 (4) ~ (O=C--N)

476 (m) .. 470 (2) (CHa--N--CHa)

422 (m) .. 422 (3) r (C--CHa) .. 262 (2) r (CHa--N--CHa) Vibrations of N, N-Dhnethylformamide & N, N-Dimethylacetamide 113

FREOUENCY (Cm "l)

4000 2500 2000 16oo ~4oo 9200 ,oo ~ooo 900 eoo 700 0'0- ~ I I 1 I I 1

I '~== 0,2 n-ID O r 0-4, 0.6- 0'8- A i.,~ " i i '4000 2500 2000 1600 1400 1200 I100 t000 900 800 700 "~ I ! I ! l, I ,, I ,1 I I I 0,0-

hi u Z 0.2 en O: 2-" 0 ~n 0.4 m 0.6~ 0.8- ~ I'5" Fto. 1. Infrared spectra of (A) Dimethylformamidv, (B) Dimethylformamide in carbon tetra-chloride. FREQUENCY (Cm -I) 40oo 2~0o zooo moo f4oo Izoo luoo iooo 9oo eoo 7oo ! I I I I, ! I ! I I I 1 ! 0-O-

Z 0.2- ID

~ 0.4- '~ 0,6- 0.8- - A 1.5- 40O0 2500 2000 1600 1400 1200 AO0 I000 900 800 700 I I I I ~ I i 1 I. I I 0"0 I

~J (J Z 0.2 < m r~ ~ 0.4

'~ 0"6- 0.8- B I'5- ..... J

Fin, 2. Infrared spectra of (A) Dimothylar162 (13) Dim162162 in carbol~ t162 114 V. VENKATA CHALAPATHI AND K. VENKAIA RAMIAH

III. •OgMAL CO-OgDINATE TREATMENT

The structures of N, N-dimcthylformamide and N, N-dimethylacetamide are shown in Fig. 5. The two tertiary amide molecules are treated as six-body problems taking each CH3 group as a peint mass. These molecules belong to the point group Cs and therefore the twelve fundamental frequencies are classified into 9 in-plane (A') and three out-of-plane (A") vibrations. The ortho- normalised set of symmetry co-ordinates for the in-plane vibrations of N, N-dimethylformamide are given in Table III and similar expressions aro used in case of the other molecule.

T.~BLE III Symmetry co-ordinates for the in-plane vibrations of N, N-dimethylformamide

Symmetry co-ordinates Vibrational ruede

R~ : I/V'2 (Ad-- Ae) .. N--CHa asymmetric stretching R2 ----- 1/~/2 (Ad + Ae) .. N--CHa symmetric stretching

Ra ----- Ac .. C--H stretching

R4 ---- Ab .. C=O stretching R5 -- Aa .. C--N stretching Re = 1/~/6 (2Ade --/kad-- Aae) .. CH3--N--CH3bending R~ = l/x/2 (Aac -- Abc) .. C--H deformation R8 = 1/~/2 (Aad-- Aae) .. CHa--N--CHa rocking Ra = 1/~/6 (2Aab-- Aae- Abc) .. O=C--N bending Rl0 = 1/~/3 (Aad+ Aae + Ade) .. Redundant Ru = 1/~/3 (Aac + Abe + Aab) .. Redundant

The elements of the F-matrix have been obtained from the expression

F ---- uf• (1) using general quadratic poterttial energy matrix containing the valence forces and their interactions. The elements of the G-matrix have been Vibrations of N, N-Dimethylformamide & N, N-Dimethylacetamide 115

derived using Decius tables, x~ The structure parameters used in these cal- ctalations are given in Table IV.

TABLE IV Structure parameters

N, N-Dimethylformamide N, N-Dimethylacetamide Bond Inter-bond Bond Inter-bond distanees angles distanees angles r(C=O)=l.23A O=C--N =120 ~ r(C=O) =1"23A O=C--N =123 ~ r (C--N) = 1.29 A N--C--H = 120 ~ r (C--N) = 1-29 A N--C--CHa =117 ~ r(C--H) = 1.07A H--C=O = 120 ~ r(C--C) = 1"55 A CHa--C=O =120 ~ r (NmC) = 1.47 A C--N--CHa = 120 ~ r (N--C) = 1"47 A C--N--CHa =120 ~ CH3--N--CH8 = 120 ~ CH3--N--CH3= 120 ~

The calculations of normal vibrations of these tertiary amide molecules were made by the method of Wilson. a7,18 For the solution of the secular determina•t which has the dimensionality of nine by nine, a programme was written in Fortran language on the basis of the modified Danielewsky's method19 for Model 2, IBM, 1620 digital computer to compute the polynomial coefficients. IBM 1620 general programme library file 1~o. 7.0.032 was used to extract the roots of the polynomial. At first the force constants were transferred from allied molecules7,a~ and alterations were made in them to obtain a close fit between the observed and calculated frequencies in case of N, N-dimethylformamide. Of the fifteen force constants, ten force constants were transferred from N, N-dimethylformamide to N, N- dimethylacetamide and only two ccnstants were to be slightly altered to get a close fit between the observed and calculated frequencies. The rest of the tire force constants are related to the stretching, bending and interaction constants of the C--H group in 1~, l'q-dimethylformamide and these are naturally different from those related to the C--CH3 group in N, •-dimethylacetamide. The final sets of force constants used in the numerical computations are given in Table V. The stretching force constants of C=O and C--N bonds haveintermediate values of the force constants which one expects from the idealised double 116 V. VENKATA CHALAPATHI AND K. VENKATA RAM/AH

TABLE V Force comtants of N, N-dimethylformamide and N, N-dimethylacetamide

Amide Type of force constant N, N-dimethyl N, N-dimethyl formamide acetamide

Bond stretching .. fa =f, =4.7 fa =f, =4"7 f, =4.5 A =3"o f~ =9.0 f~ -----9.0 f~ =6.8 f, =6-8 f,~=l.5 f,~=l.5 f~~ = 0.9 fb, -----0.9 Bond bending La :0"54 f,, ----0"8 fo, =fa, = 0"80 f,a =f,, = 0"9 f~ = 0"85 foa = 1 "2 fea =0"1 f,' =0"1 Bond/bond fo~ = 0"8 f.~ = 0"8 A* =0"4 A* = 1"2 f.' = 1.8 fo' = 1.2 f,'" = 0.3 fo~ = 0'6 Bond/angle f,~' = 0"9 f~~~ ---- 0"9 bond and single bond linkages of these atoms, indicating partial double and single bond characters for these linkages in mides. Although the

V:s k \CHa/frequencies are almost the same in both the amides, the frequencies arising out of the v., Ik. \Crin/' 7(CHa--N--CHa)znd ~(o=c--1~ vibrations are considerably di¡ in the two amidr But the corrr strr and bending forcr constants fe, fa, fa, fb, fab, fbc havr samr valur and faa, fae and fea ate only slightly differr Vibrations of N, N-Dimethylformamide & N, N-Dimethylacetamide 117

[V. POTENTIAL •NERGY DISTRIBUTIONS AND OF THE ABSORPTION FREQUENCIES

The elem~nts of the L-matrix in case of each molecule have been eva- luated by a set of homogeneous equations, represented by (GF -- Eak) ak = 0 (2)

where ak is the k-th column of matrix 'a' and the normalisation factor Nk is obtained from Nk ~ ~, Ftt, atk at,k = ak. (3) t~ t The elements of the L-matrix have been obtained by multiplying each column ak by the corresponding normalisation factor Nk. The elements of the L-matrices ate thus normalised to LI~ = G. Using the expression (FitLis2/~s) x 100, the potcntial energy distribution of each normal mode among the various symmetry co-ordinates is calculated and given in Tables VI and VII. TABLE VI The observed and calculated frequencies and potential energy distribution of different normal modes of vibrations of N, N-dimethylformamide

Frequency P.E.D. Mode of vibration Obser. Cal. R1 R2 Ra R4 R5 R6 R~ Ra Rg

v,(CHa--N--CH3) .. 1256 1252 48 10 2 18 14 4 0 4 1 v,(CHa--N--CH3) .. 865 888 23 35 0 5 3 4 4 0 29 v(C--H) .. 2870 2882 0 0 90 1 0 0 2 0 0

v(C=O) .. 1685 1704 3 0 12 45 8 0 41 4 4 v(C--N) .. 1499 1510 11 3 16 3 32 1 16 6 36

(CH3--N--CH3) .. 400 418 2 2 1 1 9 85 0 1 1 .. 1381 1345 3 8 0 25 27 3 46 0 0

(CH3--N--CH3) .. 320 304 3 0 0 0 2 5 0 75 13

.. 662 661 8 41 2 5 20 5 0 2 18 ] 18 V. VENKATA CHALAPATttI AND K. VENKATA RAMIAH

MICRONS 15.0 20.0 15.0 20.0 25.0 0.0 I - ,,

.10 tu .20 (.} Z ,ti ~ .30 O m .40 .50 ,60 .70

t.0

.... I [ I 1 l ..i i 900 800 700 600 500 800 ' 700 600 500 400 FREQUENCY (cm-I ) FIo. 3. Infrared spectra of (A)DimothyIformamidc, (B)D~mr

0 CH3

i\\ \ o {/\ bc C-"}'-="~L-N ,d

H CH N,N- DIME THYLFORMAMIDE :3 / CH3

0•• O /~C" N.'x H3 C CH3 N,N- DIMETHYLAr FIO, 5, Structurr of Dimothylformamide and Dimcthylacvtamidr Vibrations of N, N-Dimethylformamide & N, N-Dimethylacetamide 119

T•BLE VII The observed and calculated frequencies and potential energy distribution of different normal modes of vibrations of N, N.dimethylacetamide

Frequency P.E.D. Mode of vibration Obser. Cal. R~ R2 Ra R4 R5 Re R7 Ra R9 v,, (CHa--N--CHa) .. 1263 1254 37 12 1 28 11 6 0 3 1 v, (CHa--N--CHa) .. 738 726 2 40 29 4 5 0 5 2 2 v (C--CHa) .. 957 986 26 21 26 0 2 6 4 0 34 v (C=0) .. 1657 1662 3 1 2 63 21 1 6 5 7 v (C--N) .. 1494 1504 17 5 5 0 39 4 1 8 35

~,, (CHa--N--CHa) .. 476 503 8 3 18 9 5 37 20 9 2 y (C--CHa) .. 422 381 2 0 3 1 12 32 29 16 5

~, (CHa--N--CHa) .. 262 258 1 4 0 0 1 0 32 56 10 (o =c--N) .. 593 605 3 17 40 0 11 14 4 0 10

Tables VI and Vil also give the observed and calculated frequencies.

The bands at 1256 cm. -1 and 865 cm. -1 in dimethylformamide or at 1263 cm. -x and 738 cm. -1 in dimethylacetamide are due to the asymmetric

,CH3 and symmetric stretching vibrations of the N( group. In both the \ CH3 molecules, to the asymmetric stretching frequency of the N--(CHa)z group, contributlon comes from the symmetric stretch of the same group and also fromv (C=O) and v (C--N) vibrations. On the other hand, to the sym- met¡ stretching vibration of the N--(CH3)2 in dimethylformamide, a considerable contribution is from the asymmetric stretching vibration of the same group, but in dimethylacetamide v(C--CHa) vibration makes substantial corttribution to this absorption. This may explain for the difference in the symmetric stretching frequencies of the N--(CHa)~ group in these two molecules. 120 V. VENKATA CHALAPA]I-II AND K. VENKATA RAMIAI-I

The band at 2870 cm.-1 in dimethylformamide is due to the v (C--H) vibration and the potential energy distribution shows that the contribution of other modes to this one is almost negligible. Such results are characteri- stic of stretching vibrations of C--H and N--H linkages? ,6 The corresponding stretching vibration in dimethylacetamide is the v(C--CH3) vibration at 957 cm.-1 The contributions from the I~I--CH3 stretching and OCN bending vibrations to this mode are considerable.

The band at 1657cm.-1 in dimethylacetamide or at 1685cm_ 1 in dimethylformamide is essentially due to the C~=O .stretching vibration. To the band at 1657cm.-z in dimethylacetamide, there is a considerable contribution from C--I',I stretch and this result is similar to that of Suzuki s in case of acetamide. But in dimethylfo~mamide, to the C=O strctch at 1685 cm.-x, a substantial contribution is from (CH) vibration and the contribution from C--N stretch is relatively low. The normal co-ordinate treatment of formamide molectde by Suzuki5 also shows that the (CH) vibration contributes substantially to v(C=O) apart from the v(CmN) vibration.

The normal co-ordinate treatment of primary amidc molecuks5,6 shows that the band in the region of 1350 cm.-1 to 1300 cm.-z is the C--N stretch. In secondary amides, 9 the bands in the regions of 1550 cm.-z and 1300 cm.-z to 1250 cm.-z are assigned as due to the combined contribution of 3 (NH) and v (C--N) vibrations. But a band in the region of 1500 cm.-~ is assigned to C--/q stretching vibration in tertiary amides and the potenfial energy distribution supports this assignment. This result is strikingly different from that in primary and secondary amides and is in agrecment with the assignment of Lumley Jones made on the basis of band contour studies of infrared absorption bands.

The band at 1381 in dimethylformamide is due to b (CH) vibration and the C-----O and the C.--N stretches contribute to this frequency. Ir is of interest to note from the potential energy distribution that the deformation CH vibration, likewise contributes to the C=O and the C--N stretches. The bands at 405 cm.-z and 316 cm. -z in dimethylformamide are due to the bending and rocking modes of vibrations of N--(CH3)~ group and the contributions of other modes to these two bands are small. Eut in dimethyl- aeetamide, the bands at 476 cm.-1, 262 cm.-1 and 422 cm.-a are assigned to the bending, rocking vibrations of 1N--(CHs)2 group and the rocking vibration of C--CH3 group respectively and the potential distributions indicate considerable intcraction betwccD these thrcc modes of vibrations. This Vibrations of N, N-Dimethylformamide & N, N-Dimethylacetamide 121 may explain for the diffcrcnce in the bcnding fiequencies of N--(CH3)2 group in dimethylformamide and dimethylacetamide.

Ithas already becn mentioned that Lt:mley Jones assigned the frequencies in the region 3100 cm.-1 to 500 cm. -1 on the basis of band contour studies. The assignments made by the authors of the in-plane vibrational frequencies in the same region on the basis of the normal co-ordinate trealment are in agreement with those of Lumley Jones. In addition, the normal vibrational studies enablcd the authors to study the nature of each absorption band and a~sign th ~. low fr:quencies in the region 500 cm.-1 to 250 cm.-1 for the filst time to the various bending modes of vibrations.

ACKNO WLEDGEMENT$ We are grateful to Dr. G. S. Sidhu, Director, Regional Research Laboratory, for giving us facilities to work with the computer. We also thank Mr. P. Jagan Mohan Reddy for his help in the use of the computer in the numerical computations. One of us (V. V. C.) is grateful to the Council of Scienti¡ and Industrial Research, Government of India, for the award of a Senior Research Fellowship.

REFERENCES

1. Evans, L C. .. J. Chem. Phys., 1954, 22, 1228. 2. Darles, M. and Evans, J.C. lbid., 1952, 20, 342. 3. Puranik, P.G. and J. Afol. Spectroscopy, 1959, 3, 486. Vcnkata Ramiah, K.

4. Proc. lnd. Acad. Sci., 1961, 52, 69. 5. Isao Suzuki .. Bull. Chem. Soc. Japan, 1960, 33, 1959.

6, - lbid., 1962, 35, 1280. 7. Puranik, P. G. and Proc. lnd. Acad. Sci., 1962, 56, 115. La.litha Sir Doshmukh 8. Tatsuo Miyazawa, J. Chem. Phys., 1956, 24, 408. Takehiko Shimanouchi and San-lchiro Mizhushima 9. .. Ibid., 1958, 29, 611. 10. Venkata Chalapathi, V. and Proc. lnd. Acad. Sci., 1966, 64, 148. Vonkata Ramiah, K. 11. Isao Suzuki .. Bull. Chem. Soc. Japan, 1962, 35, 540. 122 V. VENKATA CHALAPA'IHI AND K. VENKATA RAMIAH

12. Bradbury, E. M. and Spectrochim. Acta, 1963, 19, 995. Elliot, A. 13. SoichiHayashi .. J. Chem. Soc. Japan, 1965, 86, 790. 14. 3"ones,R. L. .. J. Mol. Spectroscopy, 1963, 11, 411. 15. Katon, J. E. .. Ann. Chem., 1964, 36, 2126. 16. De~ius,3. C. .. J. Chem. Phys., 1948, 16, 1025. 17. Wilson,E. B. .. Ibid., 1939, 7, 1047. 18. .. Ibid., 1941, 9, 196. 19. Danielewsky, A. .. Trans. Moscow. Maths. Soc., 1937, 44, 169. II. Venkata Chalapathi and Proc. lnd. Ar Sci., A, Vol. LXVIII, Pl. V K. Venkata Ramiah

(A)

(B)

F16. 4. Raman spectra of (A) Dimethylformamide, (B) Dimethylacetamide.