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
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