CHAPTER 2

CHAPTER 2

VIBRATIONAL SPECTRA AND NORMAL COORDINATE ANALYSIS OF N,N DlMETWACETAMlDE

2.1 I DUCTI ION

As amldes are the slmplest model for peptldes, thelr exact

structure has been the subJect of many experimental and

theoretical studles. A systematlc study on the vlbratlonal

spectra of prlmary, secondary and tertiary amldes recelved

conslderable attentlon In the spectroscopic literature In vlew

of their obvlous lmportance to blologlcal systems. Studles of

lnternolecular assoclatlons, dlchrolc absorptlon, band contour

of the vapour spectra. measurement of lntegrated lntensltles of

the absorptlan bands and normal coordlnate analysls gave

lnformatlon regarding the nature of the functional groups.

orbltal lnteractlons and mlxlng of skeletal frequencies.

The vlbratlonal spectra of primary 11-31 and secondary amldea

14-61 and of thelr deuterated specles In dlfferent state of

aggregation were reported earller and the normal coordlnata

analysls of son of them uldes were also carried out. The

results obtalned from these mtudles revealed that all uldes

show a carbonyl absorption band. termed as amlde 1 band and Its

posltlon depends on the physlul st.te of the compound [71.

In prlmary uldes, the amlde I. amlde I1 and amlde 111 b u d s

are ulnly attrlbuted to CIO mtretchlng. NH2 deforutlon ud

C-N stretchlng vlbratlons,respectlvely. But in secondary

amldes. the amlde I 1 and amide I 1 1 bands are due to the

comblned contrlbutlon of N-H deformation and C-N stretchlng

vlbratlons.

As far as the vlbratlonal spectra of tertiary amldes are

concerned. Lumely Jones 181 flrst asslgned the fundamental

vlbratlonal modes on the basls of band contour studles of

lnfrared absorptlon ba nds. Then. Katon 191 attempted to

lnterpret the nature of vlbratlonal frequencles in the reglon

700 - 250 cm-l. But. the lnltial work on tertlary amldes uslng

normal coordlnate treatment to Investigate the nature of the

vlbratlonal frequencies was done by Venkatachalapathl et a1

1101. Thereafter. the studles on Raman and lnfrared spectra of

tertlary amldes through normal coordlnate treatment have

become the central Issue of several research papers.

Venkatachalapathy and co workers reported the analysis on the

vlbratlonal spectra of N.N-dlmethylformamlde 1101. N.N-

dlmethylacetamlde 1101. N.Ndlmethylthloformamlde Ill1 and

N.N-dlmethylthloacetanlde I111 by treatlng methyl groups as

polnt mass. Durgaprasad el a1 1121 reported the Infrared

speclra of N.N-dlmotylformamlde and also they explalned the

normal vlbratlons by adoptlng Urey-Bradley Force Fleld (UBFF).

Anthonl and co workers I131 asslgned the fundamental vlbratlons

of N.N-dl~thylselenoformamlde (WSF) uslng General Valence

Force Fleld (GVFF).

Although numerous lnvestlgatlons have been made on the above

tertlary amldes,stlll there are some amblgultles In the

vlbratlonal asslgnments as well as force flelds. Vlbratlonal

coupllng whlch 1s promlnent In prlmary and secondary amldes was

also not consldered serlous In most of the earller work on

tertlary amldes. To study the nature of the vlbratlonal

frequencles and to develop a more plauslble and conclse set of

force constants of some tertlary amldes. N.N-dlmethylacetamlde

(OHA) has been chosen for lnvestlgatlon In thls chapter. DHA

1s wldely used as solvent for plastics. resins. gums and high

purlty solvent for crystalllzatlon and purlflcatlon.

Venkatachalapathy et a1 1101 reported the lnfrared and Raman

spectra of DHA In the reglon 3100-250 cm-I and obtalned a GVFF

by treatlng methyl groups as point mass. Pa) was reported only

for the In-plane vlbratlonal modes. The normal coordlnate

analysls on DHA 1141 was consldered incomplete as It could not

explain the nature of the mlxing of out-of-plane vlbratlonal

modes. Thus. In order to make complete vlbratlonal asslgnments

for MA. Raman and lnfrared spactra have been recorded and

normal coordlnate analysls has been carrled out here. The

results obtalned In thls analysls has been found useful for

lnvestlgatlon on the vlbratlonal spectra of other related

tertlary amldes chosen In thls thesls.

2.2 EXPERI nENTAL

DM& was obtalned from S. d. Fine Chem. Ltd.. Bombay. The Raman

spectrum of DMA was recorded In the region 4000-100 cm-I

on a Dllor 224 Raman Spectrometer equlpped wlth a Spectra

Physlcs Model 165 argon-Ion laser source operating on 488 nm

llne wlth 200 mw power. The spectrum was recorded wlth a

scannlng speed of 30 cm-I 6th-I with a spectral width,

2.0 cm-l. The frequencies for all sharp bands were

accurate to i 2 cm-l. The FTIR spectrum of DHA was recorded

In the reglon 4000-100 cm-I on Shlaadzu FTIR 8101

spectrophotometer.

2.3 NORMAL COORDINATE ~ A ~ N T

The molecular model and Internal coordlnates adopted for the

normal coordlnate calculation Is shown In Flgure 2.1, in which

the torsional and wagglng coordlnates are not shown. From the

structural polnt of vlew, thls molecule belongs to C

symmetry and the 39 normal modes of vlbratlon are dlstrlbuted

as 24 In-plane (a') and 15 out-of plane (a*) type. The method

of Wllson 1151 was used to perform the normal coordlnate

calculations ulth the ald of Schachtschnelder's program I161.

The structural parameters employed In thls calculation are

shown In Table 2.1 and the symmetry coordlnates are glven In

Table 2.2.

The normal coordlnate treatment of M A uslng modlfled UBFF by

FIG. 2.2. FOURIER TRANSFORM INFRARED SPECTRUM OF N, N-OIMETHEYLACETAMIDE

hrrgaprasad et a1 1141 was nelther complete nor entlrely

satlsfactory as the UBFF type of fleld was found lnsufflcient

to descrlbe all the lnteractlons In a molecule. But. SGVFF

has k e n shown to be very effectlve In the normal

coordlnate analysls of arldes 1171 and also. the valence

force constants can be transferred between the related

molecules uhlch Is found very useful In the normal

coordlnate analysls of large polyatomlc molecules. Hence, in

thls work. SCVFF has been employed to express the potentlal

energy. The force constants used In the case of

N.N-dlmethylformamlde I101 and N-methylformamlde 1181 were

transferred and sllght alteratlons were made In a feu

constants to obtaln a close flt between the observed and

calculated frequencles of DMA. Thls Set of force constants Is

subsequently reflned by damped least square technique.

keeplng few lnteractlon constants flxed throughout the

reflnement process.

2.4 RESULTS AND DISCUSSIONS

The observed F T l R and laser Raman spectra of OEU are shown In

Flgures 2.2 and 2.3. The lnltlal and flnal set of force

constants used for the analysls are glven In Table 2.3. PED

along wlth the observed and experlaental frequencles are

presented In Table 2.4. The vlbratlonal assignments for all

the ln-plane and out-of plane vlbratlons are made by referrlng

to the posltlon of the correspondlng bands In related tertlary

amldes and from the PED obtalned In thls analysls.

The results of the normal coordlnate analysls on DHA

predlcts two out-of-plane asymmetrlc C-H stretchlng modes In

(CH312 lylng In a small Interval of 3 cm-l and two other

In-plane modes at 3038 and 3040 cm-l. Slmllarly. the two

fundamentals due to ln-plane symmetrlc stretchlng are almost

colncldent near 2860 cm-l. Hence, the Raaan band observed

at 2932 and 3042 cm-I have been each doubly asslgned to the

out-of plane asymmetrlc and ln-plane asyluatrlc C-H stretchlng

mode In (CH312 . The fundamental due to a' asymmetry stretch of

C-H In CH3 has been asslgned to the medlum Intenslty band

observed at 2980 cm-I in lnfrared. The 111 resolved shoulder

observed at 2820 cm-I in the lnfrared spectrum of DMA has been

asslgned to a' symmetry stretch of C-H In CH3 and the above

assignments are In close agreement wlth the earller llterature

values 110-141.

The very strong lntenslty lnfrared band observed at 1657 cm-I

In the lnfrared spectrum of DHA and the correspondlng strong

lntenslty Raman band at 1648 cm-I 1s due to CIO stretchlng

coupled wlth C-N stretchlng mode. The posltlon of thls band

agrees well wlth that of amlda I band asslgned for the related

tertlary auldes 111.12.141.

As has been polnted out by earller workers 110.141. one of the

dlfflcultles In lnterpretlng the vlbratlonal spectra of DMA 1s

the conplexlty of spectra In the range 1300-1500 cm-l. However,

the present calculation glves a good account of the nature of

these bands. The bands observed In the reglon 1300-1500 cm-'

are due to the deformatlonal modes of methyl groups. The

avallable spectra shows nlne bands In that reglon. The medlum

lntenslty lnfrared band observed at 1464 and 1397 cm-l have

been asslgned to asymetrlc and synmetrlc CH3 deformation of

a' specles. The weak band observed at 1426 and 1420 cm-I in

lnfrared are due to asymmetrlc (CH3)2 deformatlonal modes of

a' specles. It mixes wlth the C-N stretch and methyl group

stretchlng vlbratlons. Depending on the PED and the calculated

values of frequencles, the lnfrared band observed at 1354 and

1440 cm-I have been each doubly asslgned to syuetrlc (a') and

asymmetrlc (a') (CH3I2 deformatlonal modes. The remalnlng a'

asymmetrlc deformatlonal mode Is ldentlfled at 1430 cm-I In

the lnfrared spectrum of MA. The above asslgments are In

close agreement wlth the related structures [lo-141.

The symetrlc and asymaetrlc stretchlng vlbratlons of ~{:3 are 3

asslgned by taklng Into account. thelr characterlstlc

lnterchange of lntenslty In lnfrared and Raman. Thus, the

strong lnfrared band near 1260 cm-I and the relatlvely weak

band around 738 cm-l Whlch have relatlvely weak and strong

Roman counterparts at 1259 and 747 cm-I respectlvely, are

asslgned to asymmetrlc and symmetric N-C stretchlng modes of

~<:3 . The above asslgnments are sllghtly different from 3

Durgaprasad et a1 1141 vhlch 1s obvlously due to the mixlng

of rocklng and deformatlonal modes of dlmethyl groups ulth thls

mode.

The medlum band observed at 1058 cm-I and the strong band at

1132 cm-I In lnfrared are each doubly asslgned to a' and a"

rocklng modes of dlmethyl groups. Accordlng to normal

coordinate analysls, the bands at 1020 and 1145 In

lnfrared have been asslgned to CH, rocklng modes of a' and a'

specles. The band observed at 959 cm-I in lnfrared 1s

asslgned to C-C stretching. The medlum band observed at

597 cm-I In lnfrared has been asslgned to O=C-N ln-plane

bendlng and 1s In close agreement ulth Durgaprasad et a1

(141. The band observed at 478 c i l has the asslgment of

coupled C-N-C bendlng and deformatlon modes of CH3 as

expected.

In the low frequency reglon, the calculated frequencles. 229,

155. 120 and 114 cm-I correspondlng to the Raman bands at 230,

152. 132 and 120 cm-I arlse due to the out-of plane torslonal

modes of C-N and methyl groups. The remalnlng vlbratlonal

asslgnments ulth PED are presented In Table 2.4.

TABLE 2.1

Structure paru~eters of N,N-dlmethylacetamide

Bond distances Bond angles

C-0 = 1.23 A'

C-N = 1.29 A *

C-C = 1.54 A'

N-C 1.46 A'

C-H = 1.10 A *

O=C-N = 123'

N-C-C = 117'

C-C=O = 120'

C-N-C = 120'

C-N-C = 120'

ti-C-H = 109~47'

TABLE 2.2

Symctry coordlnatss for N.N-dimethylacetarnids

In plans vibrations

(12)-'/' (2Aq-As-Att2Au-Av-Awl asym C-H stretch In (CH3I2

( 1 2 1 - ~ / ~ (2Aq-As-At+2AutAv-Au) asym C-H stretch In (CH3I2

(3)-'/' (Ap-Am-An) asym C-H stretch In CH3

( 6 ) - 1 ~ 2 ~ ~ q + ~ s + ~ t + ~ u + ~ v t ~ ~ ) sym C-H stretch in (CH3I2

(6)-l/' (Aq+As+At-Au-Av-Au) sym C-H stretch In (CH3I2

(3)-l/' (Ap+Am+An) sym C-H stretch in CH3

Ar C=O stretch

AD C-N stretch

( 3 )-l/' (Apm-Amn-Anp) asym CH3 deformatlon

( l ~ ) - l / ~ (2Aqs-Ast-Atq +2Auv-Avu-Auu) asym (CH3IZ deiormatlon

(2Aqs-bst-Atq

-2Auv+Avu+Awu) asym (CH312 deformatlon

(6 )-l" lApm+Amn+Anp -ARp-Ah-ARn sym CH deformation 3

(12)-1/2 (Aqs+Ast+Atq-Aeq-Aes -Aet+Auv+Avu+Auu-Adu-Adv-d) sym (CH deiormatlon 3 2

(1~1-l'~ (Aqs+Ast+Atq-Aeq-Aes-Aet -Auv-Avu-Auu+Adu+AdvtAdu) sym (CH ) deformatlon 3 2

(2)-'I' (As-Ad) asym N<'% stretching CH

S16- (6)-I/2 (2Ade-AW-ADe) asyn ~<"3 deformatlon CH

SI7- (12)-1'2(ZAeq-~es-~et+2~du

-Adv-Adw (CH312 rocklng

S18= (12)-1'2(2Aeq-~es-~et-2~du

t~dv+ddw) (CH3I2 rocklng

S19- ( 6 ) - 1 / 2 ( Z A ~ p - ~ ~ m - ~ ~ n ) CH3 rocklng

SZO= AR C-C stretchlng

sym NC\ stretchlng cH3

SZ2= (6)-I/' (2ADr-ArR-ARD) O=C-N bendlng

Sz3= 16)-l/~ (2Ade-Aed-ADd) C-N-C bendlng

~ < " 3 rocking

CH3

Out of plane vibrations

(4 )-I/' (As-Aqtbd-Au 1

(4 (AS-~q-AV+AU)

(2)-l/' (Ap-Am)

(q)-l/' ( ~ t ~ - b t q * b ~ u - ~ u v )

(4)-l/' (Ats-Atq-Auu+Auv)

(z)-~/' (Am-An)

(3)-I/' (ARp-Apm-Apn)

(4 )-I/' (Aeo-Aeq+Adu-Adv

(4)-'/' (bes-Aeq-Adu+Adv)

Wl

asym (Cl lg )Z stretch

asym (CH3IZ stretch

asym CH3 stretch

asym (CH3IZ deformatlon

asym (CH312 deformatlon

asym (CH3) deformatlon

CH3 rocklng

(CH31Z rocklng

(CH3I2 rocking

S12- ~1 C-N torelon

CH3 torslon

(CH3)2 torslon

(CH3Iz torslon

TABU 2.3

Force conmtsntsa of N,N-dlmothylacetamld.

TYPS of Parameter Coordlnates Inltlal Flnal

force constant Involved value value

Diagonal stretching fp

lnteractlon stretch- fDr

constants stretch f W

frR

fdd

f W

Stretch- fkd

band f W

rw fDe

C-H

C-H

C-C

C-N

N-C

CIO

HCH

HCC

NCH

tlCH

OCC

CCN

Ncu

CNC

M C

CNco

CNm

cocc NC NC

CN NC

CN CNC

CN CCN

NC CNC

Nc CNC

NC NCH

NC O E

CC NCO

NC HCH

bend- 577 NCH NCH 0.1642 -0.0217

bend fc9 CNCCNC 0.1075 0.1075

re* ~ m a c 0. 1126 0.1126

rgw ac CCN 0.0168 0.0071

f CCNNCO 0.1391 0. 1391

f7d NCH HCH 0.0025 -0.0081

fcc MCUJC 0.0091 -0.0010

All stretching force constants are In unlts of .lllldynes per angstrom, kndlng In mlllldyne angstrom per square radlans. and stretchlng-bendlng Interactions In mlllldynes per radlan.

TABLE 2.4

Observed and calculated frequencies (cm-I) and potential energy dlstrlbutlons of N,N-dl.athylacetamlde

Oberved Calculated Descrlptlon PED X 1 nf rared Raman

In plane vibrations

3036 m 3042 m 3038 asym C-H stretch in (CH3I2 S1(96)

3036 m 3042 m 3040 asym C-H stretch in (CH3I2 S2(92)

2980 m 2959 m 2948 a s p C-H stretch ln CH3 S3(92)

2870 u,sh 2867 ms 2861 sym C-H stretch in (CH3I2 S4(98)

2870 w,sh 2867 as 2860 s p C-H stretch In (CH3I2 S5(98)

2820 u,sh 2827 ms 2830 sym C-H stretch In CH S6(98)

1657 vs 1648 s 1641 C 4 stretch S7(603S8(22)

1490 m 1482 s 1484 C-N stretch S8(59)S23(10)

Sl7(10)

1464 l 1462 m 1458 a s p CH3 deformation Sg(54)Sl3l20)

Sl0(15)

1426 u 1424 u 1420 a s p (CH3)2 deformatlon Sl0148)S5(25)

1420 w 1401 a r m (CH3)Z deformation Sl1150)S,(22)

1397 r 1402 1381 s p CH3 deforutlon Sl2(7O)S8(15)

1354 l 1360 w 1344 sy. (CH3lZ deforutlon S13(64)S9(18)

1354 m 1360 w 1351 sym (CH3I2 deformatlon S14(62)S4(22) SI6(1O)

1259 m 1257 a s p ~c'3 stretchlng S15(66)S16(15)

Cu3 Sl7I10)

1182 m 1171 asyn ~ t ' a deformatlon Sl6(511Sl8(151

C"3 S15(20)

1041 (CH3I2 rocklng Sl7(45)Sl1(20)

S13(15)

1040 (CH3I2 rocklng S18(51 )S17(27)

Sl4(2O1

1018 w 1009 CH3 rocklng SI9(79)S4(10)

S7(10)

963 m 951 C-C stretching SzO(71 )SZ3(20)

747 s 735 sya N<\ stretchlng ~ ~ ~ ( 4 7 ) 5 ~ ~ ( 2 0 )

CHa S14(15)

594 m 585 04-N bendlng SI2(60)S7(17)

476 w 465 C-N-C bendlng Sz3(42)S9(20)

Out of plane vibration8

2932 s 2928 any. (CH312 stretch S1(96)

2932 u 2925 a n p (CH3I2 stretch S2(94)

2903 r 2900 u 2900 asyl (CH3) stretch S3(96)

1440 n 1432 asp (CH3I2 deformat lon S4(70)S8(253

1440 n 1429 asym (CH3I2 deformation S5(71 )S9(21)

1430 1431 asym (a3) deformation S6(74)S7(20)

1145 1136 u I140 (a3) rocklng S7(75)S6(15)

1132s 1128u 1127 (CH,)Zrocklng S8(71)S5(12)

1132 s 1128 u 1121 (CH3I2 rocklng Sg(68)S4(14)

308 n 308 301 (CH3I2 uagglng Slo(65)S2(10)

CH 3 262 u 255 N< uagglng S1 (75)

CH3

238 m 230 u 229 C-N torslon SI2(67)

148 ns 152 n 155 CH3 torslon Sl3(6O)S6(12)

116 w 120 u 114 (CH3)2 torsion S15(62)S5(16)

Abbrsvlatlons used: s, strong;m,wdlru;u,ueak;ms.medlum strong; and sh, shoulder

1. I. Suzukl. Bull. Chem. Soc. Japan, 35. 1279. 1962.

2. I.Suzuk1, Bull.Chem.Soc. Japan, 35, 1286, 1962

3. Y. Kuroda, Y. Sal to, K. Hachlda and T. Uno, Bull. Chem. Chem. Soc.

Japan, 45,2371. 1972

4. C. N. R. Rao and R. Venkataraghavan, Spectrochlm. Acta. 18. 541. 1962

6. C. N. R. Rao and C.C. Chaturvedl. Spectrochlm.Acta. 27A. 520. 1971

7. H.Sllversteln, C.Clayton Bassler and C.norll1, Spectronetrlc ldentlflcatlon of organlc compounds, John Ulley. New York, 1981

8. R. L. Jones. J. Mol.Spectrosc. 11. 411. 1963

9. J. E.Katon. Ann. Chem. 36. 2126. 1964

10. V. Venkatachalapathl and K. Venkata Ramlah, Proc. Indlan Acad. Scl. 68. 109. 1968

ll.C.A. lndlra Chary and K.Venkata Ramlah, Proc. Indlan Acad. Scl. 69. 18. 1969

12.C. Durgaprasad. D.N. Satyanarayana and C.C. Patel, Bull. Chem. Soc. Japan. 44, 316. 1971

13. U. Anthonl, L. Henrlksen and P. H. Nlelson, Spcctrochlm. Acta, 30A. 1351. 1974

14.G. Durgaprasad. D. N. Satyanarayana. C. C. Patel. H. S. Randhaw, Abha Coel and C. N.R.Rao. Spcctrochlm.Acta. 28A. 2311, 1972

16. J. H.Schachtschnelder, Technical report, Shell development co. Emeryvllle, USA, 1969

17. A.M. Dwlvedl. S. Kr l u and S. Mlerson, Spcctrochim. Acta, ISA, 271. 1989

18. I.Suzukl. Bull. Chem. Soc. Japan, 35, 540. 1962