Density functional theory study of vibrational spectra, and assignment of fundamental vibrational...

Spectrochimica Acta Part A 62 (2005) 931–939

Density functional theory study of vibrational spectra, and assignment offundamental vibrational modes of succinimide andN-bromosuccinimide

V. Krishnakumara,∗, R. John Xavierb, T. Chithambarathanuc

a Department of Physics, Periyar University, Salem 636 011, Indiab Department of Physics, J.J. College of Engineering and Technology Tiruchirappalli, Tiruchirappalli 620 009, India

c Department of Physics, S.T. Hindu College, Nagercoil 629 002, India

Received 23 November 2004; accepted 23 February 2005

Abstract

This work deals with the vibrational spectroscopy of succinimide andN-bromosuccinimide. The mid and far FTIR and FT-Raman spectrawere measured in the condensed state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using densityfunctional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis set combinations. The vibrational spectrawere interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical force field. The infrared and Ramans s importanti t of all thef©

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pectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra providenformation about the ability of the computational method to describe the vibrational modes. Unambiguous vibrational assignmenundamentals were made using the total energy distribution (TED).

2005 Published by Elsevier B.V.

eywords: Succinimide;N-bromosuccinimide; Density functional theory; FTIR; FT-Raman; Vibrational spectra

. Introduction

Succinimide and its derivatives are biologically and indus-rially useful compounds. Pharmaceutically they are useds analgesics, nephrotoxic, anticonvulsant, ionic inhibitorsf human leukocyte, etc. They are also used in industriess antifoaming agent, lubricating oil, lubricating tackifiers,mulsion explosives and also corrosion inhibitors. Someew succinimide and sulfonated derivatives were confirmed

o have analgesic activity through abdominal constrictionests in mice. It has been reported that sulfonated deriva-ives of succinimide are more effective than aspirin andaracetamol. Consideration of these factors lead to under-

ake the detailed spectral investigation of succinimide and-bromosuccinimide.Modern vibrational spectrometry has proven to be an

xceptionally powerful technique for solving many chem-stry problems. It has been extensively employed both in the

∗ Corresponding author.E-mail address: [email protected] (V. Krishnakumar).

study of chemical kinetics and chemical analysis. The plem of signal assignment, however, as well as understanthe relationship between the observed spectral featuremolecular structure, and reactivity can be difficult. Eidentification of fundamental vibrational frequencies ogenerates controversy.

Density functional theory (DFT)[1,2] has been acceptby the ab initio quantum chemistry community as a ceffective general procedure for studying physical proties of the molecules. When formulated in such a wayit resembles the Hartree–Fock theory, the self-consiKohn–Sham procedure[2] of DFT has a similar sizedependence of cost as that of the Hartree–Fock theory. Uthe Hartree–Fock theory, DFT recovers electron correlain the self-consistent Kohn–Sham procedure throughfunctionals of electron density and gives good descriptfor systems which require sophisticated treatments oftron correlation in the conventional ab initio approach[3].Although DFT was not favored by computational chemfor many years, it was recently paid much attentionmany ab initio quantum chemists and much effort has

386-1425/$ – see front matter © 2005 Published by Elsevier B.V.oi:10.1016/j.saa.2005.02.052

932 V. Krishnakumar et al. / Spectrochimica Acta Part A 62 (2005) 931–939

Fig. 1. Molecular model of succinimide andN-bromosuccinimide alongwith numbering of atoms.

devoted to refining the methodology and exploring its abil-ity [4–12]. DFT calculations of vibrational spectra of manyorganic systems[4,11,13–18], have shown promising con-formity with experimental results. Based on comparison ofthe force constants of formamide calculated by HF, MP2and DFT methods, Florian and Johnson[19] suggested thatthe SQM procedure be applied on DFT force constants tostudy vibrational problems. Later, Rauhut and Pulay[20]have shown that the raw BLYP and B3LYP frequencies andforce constants approximate the experimental results muchbetter than the Hartree–Fock results and that the use of multi-ple scale factors further improves the agreement between thecalculated and observed fundamental vibrational frequencies.

Table 1Total energies (in Hartrees) based on B3LYP/6-31G* and B3LYP/6-311+G**

basis sets for succinimide andN-bromosuccinimide

Basis set Succinimide N-bromosuccinimide

B3LYP/6-31G* −360.424 −2931.345B3LYP/6-311+G** −360.537 −2933.978

To gain a better understanding of the performance and lim-its of DFT methods as a general approach to the vibrationalproblems of organic molecules, we calculated harmonic fre-quencies of succinimide andN-bromosuccinimide by DFTmethod, and compared these results with observed funda-mental vibrational frequencies. In the present study, we reportour results on vibrational analysis of succinimide andN-bromosuccinimide using the SQM force field method basedon DFT calculation. The aim of this work is to check theperformance of B3LYP density functional force field for sim-ulation of IR and Raman spectra of the title compounds withthe use of standard 6-31G* and 6-311+G** basis sets (referredto as small and large basis set, respectively) The simulatedand observed spectra were analysed in detail.

2. Experimental

The fine polycrystalline samples of succinimide andN-bromosuccinimide were obtained from the LancasterChemical Company, UK and used as such for the spectral

Table 2Optimized geometrical parameters of succinimide andN-bromosuccinimide obtained by B3LYP/6-311+G** density functional calculation

Bond length Value (̊A) Bond angle Value (◦)

Succinimide N-bromosuccinimide Succinimide N-bromosuccinimide

N1 C2 1.459 1.463 N1C2 C3 1.352 1.350 C2C3 C4 1.559 1.549 C3C4 C5 1.352 1.350 C4C5 N1 1.460 1.461 C5C5 O6 1.430 1.429 N1N1 H7(Br7) 0.990 1.841 C2C2 O8 1.430 1.420 C5C N1C C4C C2C C2

C4C4C3C3C5C5O8O6C3C4C2C3

T

3 H9 1.060 1.0613 H10 1.070 1.0704 H11 1.070 1.0704 H12 1.070 1.070

C4

he atoms indicated in the parenthesis belong toN-bromosuccinimide. For numb

C2 C3 112.43 112.39C3 C4 105.33 105.23C4 C5 105.30 105.21C5 N1 112.40 112.32N1 C2 101.75 102.11C2 O8 123.62 123.80N1 H7 (Br7) 113.40 113.26N1 H7 (Br7) 113.41 113.26C5 O6 123.60 123.80C5 O6 123.92 123.80C3 H9 110.77 110.85C3 H10 109.52 109.52C3 H9 111.80 111.76C3 H10 110.52 110.54C4 H11 111.79 111.79C4 H12 110.43 110.48C4 H11 109.50 109.50C4 H12 111.41 111.38C2 N1 H7 (Br7) 42.36 42.36C5 N1 H7 (Br7) −42.28 −42.27C2 N1 H7 (Br7) −137.63 −137.62C5 N1 H7 (Br7) 137.67 137.67N1 C5 O6 −164.49 −164.50C4 C5 O6 169.96 169.95
C3 C2 O8 −169.93 −169.94
ering of atoms referFig. 1.

V. Krishnakumar et al. / Spectrochimica Acta Part A 62 (2005) 931–939 933

Fig. 2. Comparison of observed and calculated infrared spectra of succinimide (a) observed in solid phase; (b) calculated with B3LYP/6-31G* ; and (c) calculatedwith B3LYP/6-311G** .

Fig. 3. Comparison of observed and calculated infrared spectra ofN-bromosuccinimide (a) observed in solid phase; (b) calculated with B3LYP/6-31G* ; and(c) calculated with B3LYP/6-311G** .


Fig. 4. Comparison of observed and calculated Raman spectra of Succinimide (a) observed in solid phase; (b) calculated with B3LYP/6-31G* ; and (c) calculatedwith B3LYP/6-311G** .

Fig. 5. Comparison of observed and calculated Raman spectra ofN-bromosuccinimide (a) observed in solid phase; (b) calculated with B3LYP/6-31G* ; and (c)calculated with B3LYP/6-311G** .


measurements. The room temperature Fourier transforminfrared spectra of the title compounds were measuredin the region 4000–50 cm−1 at a resolution of±1 cm−1

using a BRUKER IFS-66V FTIR spectrometer equippedwith dual detection: a cooled MCT detector for themid-IR and a room temperature pyroelectric detectorfor the far-IR range. The samples used in these mea-surements were KBr and polyethylene pellets, respec-tively. Boxcar apodization was used for the 250 averagedinterferograms collected for both the sample and back-ground.

The FT-Raman spectra of succinimide andN-bromosuccinimide were recorded on a BRUKER IFS-66Vmodel interferometer equipped with an FRA-106 FT-Ramanaccessory in the 3500–100 cm−1 stokes region using the1064 nm line of a Nd:YAG laser for excitation operating at200 mW power. The reported wave numbers are believed tobe accurate within± 1 cm−1.

3. Computational details

The molecular geometry optimizations, energy and vibra-tional frequency calculations were carried out for succin-imide andN-bromosuccinimide with GAUSSIAN 98W soft-ware package[21] using the B3LYP functionals[22,23]cs forcec ry bya cefiu rep-r ndt leasts f thet ndR VIBpt ren-t ) of1

ingv onalm eo o thei r thev ea basisf

am[ ncya ence.T rdi-n e seta g theG

3.1. Prediction of Raman intensities

The Raman activities (Si) calculated with the GAUSSIAN-98 program and adjusted during the scaling procedure withMOLVIB were subsequently converted to relative Ramanintensities (Ii) using the following relationship derived fromthe basic theory of Raman scattering[31–33].

Ii = f (ν0 − νi)4Siνi

[1 − exp(hcνi/kT )

] (1)

whereν0 is the exciting frequency (in cm−1 units),νi is thevibrational wave number of theith normal mode,h, c andkare fundamental constants andf is a suitably chosen commonnormalization factor for all peak intensities.

4. Results and discussion

4.1. Molecular geometry

The molecular structures of succinimide andN-bromosuccinimide belong to Cs symmetry, are shown inFig. 1. The global minimum energy obtained by the DFTstructure optimization for succinimide andN-bromosucci-

TD

N

S136

11

B1

1

223

O3

3

T3

C2 C3 C4 C5,C3 C4 C5 N1,C4 C5 N1 C2,C5 N1 C2 C3

The atoms indicated in the parenthesis belong toN-bromosuccinimide. Fornumbering of atoms refer Fig 1.

ombined with the standard 6-31G* and 6-311+G** basisets. The Cartesian representation of the theoreticalonstants have been computed at optimized geometssuming Cs point group symmetry. Scaling of the foreld was performed according to the SQM procedure[15,24]sing selective scaling in the natural internal coordinateesentation[25,26]. Transformations of the force field ahe subsequent normal coordinate analysis including thequare refinement of the scaling factors, calculation ootal energy distribution (TED) and the prediction of IR aaman intensities were done on a PC with the MOLrogram (version 7.0–G77) written by Sundius[27,28]. For

he plots of simulated IR and Raman spectra, pure Loizian band shapes were used with a band width (FWHM0 cm−1.

The symmetry of the molecule was also helpful in makibrational assignments. The symmetries of the vibratiodes were determined by using the standard procedur[29]f decomposing the traces of the symmetry operation int

rreducible representations. The symmetry analysis foibrational modes of succinimide andN-bromosuccinimidre presented in some details in order to describe the

or the assignments.By combining the results of the GAUSSVIEW progr

30] with symmetry considerations, vibrational frequessignments were made with a high degree of confidhere is always some ambiguity in defining internal cooation. However, the defined coordinate forms completnd matches quite well with the motions observed usinAUSSVIEW program.

able 3efinition of internal coordinates of succinimide andN-bromosuccinimide

o. (i) Symbol Type Definition

tretching–2 Ri C–N C2 N1, C5 N1–5 ri C C C2 C3, C3 C4, C4 C5–9 Pi C H C3 H9, C3 H10, C4 H11,

C4 H120–11 Di C=O C2 O8, C5 O62 Ti N H (Br) N1 H7 (Br7)

ending3–17 βi Ring C2 N1 C5, N1 C2 C3,

C2 C3 C4, C3 C4 C5,C4 C5 N1

8–25 αi CCH C2 C3 H9, C4 C3 H9,C2 C3 H10, C4 C3 H10,C3 C4 H11, C5 C4 H11,C3 C4 H12, C5 C4 H12

6–27 γ i CNH C2 N1 H7, C5 N1 H78–29 δi CCO C3 C2 O8, C4 C5 O60–31 δi NCO N1 C2 O8, N1 C5 O6

ut-of-plane bending2–33 ωi C O O8 C2 C3 N1,

O6 C5 C4 N14 Ψ i N H (Br) H7 (Br7)–N1 C2 C5

orsion5–39 ti t Ring N1 C2 C3 C4,


Table 4Definition of local symmetry coordinates and the values of corresponding scale factors used to correct the refined B3LYP/6-31G* and B3LYP/6-311+G** forcefields for succinimide andN-bromosuccinimide

No. (i) Symbola Definitionb Scale factor used in the calculation

B3LYP/6-31G* B3LYP/6-311+G**

1–2 CN R1, R2 1.001 1.0193–5 CC r3, r4, r5 0.926 0.9516–7 CH2 ss (P6 + P7)/

√2, (P8 + P9)/

√2 0.902 0.951

8–9 CH2 ass (P6 − P7)/√

2, (P8 − P9)/√

2 0.902 0.95110–11 CO D10, D11 0.906 0.91912 NH (Br) T12 0.939 (0.941) 0.973 (0.976)13 R bend1 (β13 + a (β14 +β17) + b (β15 +β16) 0.984 0.96414 R bend2 (a − b) (β14 −β17) + (1− a) (β15 −β16) 0.984 0.96415 CH2 sciss α18 +α19 +α20 +α21 0.913 0.90116 CH2 wag α18 −α19 +α20 −α21 0.913 0.90117 CH2 rock α18 +α19 −α20 −α21 0.913 0.90118 CH2 twist α18 −α19 −α20 +α21 0.913 0.90119 CH2 sciss α22 +α23 +α24 +α25 0.913 0.90120 CH2 wag α22 −α23 +α24 −α25 0.913 0.90121 CH2 rock α22 +α23 −α24 −α25 0.913 0.90122 CH2 twist α22 −α23 −α24 +α25 0.913 0.90123 bNH (Br) (γ26 − γ27)/

√2 0.937 (0.941) 0.972 (0.977)

24–25 bCO (δ28 − δ30)/√

2, (δ29 − δ31)/√

2 0.964 0.98326–27 ωCO ω32, ω33 0.931 0.97128 ωNH (Br) ψ34 0.962 (0.969) 0.957 (0.963)29 t Rtorsion1 b (τ35 + τ39) + a (τ36 τ38) + τ37 0.931 0.82930 t Rtorsion2 (a − b) (τ39 − τ35) (1− a) (τ38 − τ36) 0.931 0.829

a = cos 144◦; b = cos 72◦,.a These symbols are used for description of normal modes by TED inTables 5 and 6.b The internal coordinates used here were defined inTable 3.

nimide are presented inTable 1. The optimized geometricalparameters obtained by the large basis set calculation (theglobal minimum energy) in this study for succinimide andN-bromosuccinimide are presented inTable 2.

Detailed description of vibrational modes can be givenby means of normal coordinate analysis (NCA). For thispurpose, the full set of 39 standard internal coordinates(both containing nine redundancies) for succinimide andN-bromosuccinimide, respectively, were defined inTable 3.From these, a non-redundant set of local symmetry coor-dinates were constructed by suitable linear combination ofinternal coordinates following the recommendations of Fog-arasi and Pulay[25,26] are summarised inTable 4. Thetheoritically calculated DFT force fields were transformedto this latter set of vibrational coordinates and used in allsubsequent calculations.

5. Vibrational spectra

The 30 normal modes of succinimide andN-bromosucci-nimide are distributed amongst the symmetry species as

Γ3N−6 = 21A′(in-plane)+ 9A′′(out-of-plane)

in agreement with Cs symmetry. All the vibrations areactive both in the Raman scattering and infrared absorp-t ntalm h

the observed and calculated frequencies, IR and Raman inten-sities and normal mode descriptions (characterized by TED)are reported inTables 5 and 6. For visual comparison, theobserved and simulated FTIR and FT-Raman spectra of suc-cinimide andN-bromosuccinimide are presented inFigs. 2–5,respectively.

Root mean square (RMS) values were obtained in thisstudy using the following expression.

RMS =√

1

(n− 1)Σni (νcalc

i − νexpi )

2

The RMS error of the frequencies (unscaled/B3LYP/6-31G* )observed for succinimide andN-bromosuccinimide are foundto be 84.10 and 89.33 cm−1, respectively. In order to repro-duce the observed frequencies, refinement of scaling factorswere applied and optimized via least square refinement algo-rithm which resulted a weighted RMS deviation of 7.2 and3.85 cm−1 for succinimide and forN-bromosuccinimide 7.36and 4.2 cm−1, respectively for small (B3LY/6-31G* ) andlarge (B3LYP/6-311G** ) basis sets between the experimen-tal and SQM frequencies. The vibrational analysis of someprominent functional groups were discussed below.

5.1. Methylene group vibrations

i-c

ion. The detailed vibrational assignments of fundameodes of succinimide andN-bromosuccinimide along wit

For the assignments of CH2 group frequencies, basally six fundamentals can be associated to each CH2 group


Table 5Assignment of fundamental vibrations of succinimide by normal mode analysis based on SQM force field calculations using selectively scaled B3LYP/6-311+G**

force field

No. Symmetryspecies Cs

Observed fundamentals Calculated frequencies (cm−1) TED (%) among types ofinternal coordinatesd

FTIR FT-Raman Unscaled Scaleda Aib Ii

c

B3LYP/6-31G*

B3LYP/6-31G*

B3LYP/6-311+G**

1 A′ 3405 w 3566 3412 3409 0.289 1.09 NH (100)2 A′ 3006 vw 3159 3012 3009 0.516 0.89 CH2 ass (98)3 A′ 2997 vw 2993 w 3146 2992 2999 0.299 6.21 CH2 ass (98)4 A′ 2973 m 3109 2965 2977 0.262 29.46 CH2 ss (96)5 A′ 2810 w 2951 2815 2813 0.194 4.11 CH2 ss (98)6 A′ 1768 s 1753 m 1896 1773 1770 0.513 25.48 CO (88)7 A′ 1701 vs 1822 1711 1707 0.707 0.89 CO (86)8 A′ 1695 w 1814 1699 1697 0.863 0.92 CC (85)9 A′ 1498 w 1609 1500 1498 0.215 3.37 CC (87)10 A′ 1475 vw 1547 1482 1472 0.351 2.46 CH2 sciss (86)11 A′ 1429 m 1433 w 1498 1441 1436 0.473 4.38 CH2 sciss (84)12 A′′ 1395 w 1466 1386 1401 0.316 0.76 CH2 wag (71)13 A′′ 1374 w 1438 1383 1370 0.742 1.14 CH2 wag (73)14 A′ 1348 m 1408 1352 1346 0.408 40.26 CN (83), CC (12)15 A′ 1316 w 1374 1323 1319 0.392 23.18 CN (85), CC (14)16 A′′ 1294 vs 1343 1288 1298 0.521 2.42 CH2 twist (61)17 A′′ 1250 w 1294 1242 1252 0.643 4.61 CH2 twist (63)18 A′ 1212 w 1252 1216 1210 0.496 8.42 bNH (74), CN (12)19 A′ 1196 vs 1234 1198 1195 0.329 1.12 CC (80), CN (12)20 A′ 1030 ms 1058 1033 1028 0.363 1.71 CH2 rock (71)21 A′ 1003 ms 1026 1013 1009 0.326 3.21 CH2 rock (69)22 A′ 935 vw 952 943 932 0.218 0.92 R bend 1 (61), bNH (16)23 A′ 851 ms 870 844 856 0.374 2.42 R bend 2 (63), R bend 1 (23)24 A′ 668 vs 689 672 666 0.619 0.72 bCO (63), CC (21)25 A′ 636 s 652 641 633 0.634 62.39 bCO (61), CC (19)26 A′′ 477 vw 489 483 479 0.226 0.91 �CO (57), bCO (21)27 A′′ 422 vw 443 428 420 0.557 3.01 �CO (54), CN (19)28 A′′ 368 w 387 378 362 0.134 1.02 �NH (52), CN (23)29 A′′ 241 w 261 250 243 0.112 0.51 t Rtorsion 1 (56),�CO (21)30 A′′ 189 w 206 200 196 0.098 1.24 t Rtorsion 2 (53),�CO (19)

a For optimised values of scale factors seeTable 4.b Relative absorption intensities normalized with highest peak absorbance equal to 1.0.c Relative Raman intensities calculated by Eq.(1) and normalised to 100.d For the notation used seeTable 3. Abbreviations used: ss: symmetric stretching; ass: antisymmetric stretching; b: bending; R: ring; t: torsion;�: out-of-plane

bending; vs: very strong; s: strong; ms: medium strong; m: medium; w: weak; vw: very weak.

namely, CH2ss—symmetric stretch; CH2ass—antisymmet-ric stretch; CH2 sciss—scissoring and CH2 rock—rockingwhich belongs to in-plane (A′) vibrations. In addition to that,CH2 wag—wagging and CH2 twist—twisting modes of CH2group would be expected to be depolarised for out-of-plane(A′′) symmetry species.

The antisymmetric CH2 stretching vibrations are gen-erally observed in the region 3100–3000 cm−1, while thesymmetric stretch will appear between 3000 and 2900 cm−1

[34–37]. The CH2 antisymmetric vibrations were observed inIR and Raman spectra at 3006, 2997 and 2993 cm−1, respec-tively, for succinimide and forN-bromosuccinimide at 3009,2998 and 3000 cm−1, respectively. The symmetric stretchingwere also identified in Raman and IR spectra at 2973 and2810 cm−1 for succinimide and 2982 and 2912 cm−1 for N-bromosuccinimide. The bands corresponding to scissoring,wagging, rocking and twisting vibrations of CH2 group are

summarised inTables 5 and 6. These assignments are alsosupported by TED output.

5.2. Carbonyl absorptions

Carbonyl absorptions are sensitive and both the carbonand oxygen atoms of the carbonyl group move during thevibration and they have nearly equal amplitudes. When twocarbonyl groups are in the compound, then the frequency ofinteractions between them are possible. The extent to whichfrequency coupling effects between two carbonyl groupsoccur depends upon a number of factors. These includes:(i) the proximity of the two groups; (ii) the angle between thecarbonyl groups; and (iii) the degree to which the individualfrequencies approach each other.

Normally carbonyl group vibrations[38,39] occur in theregion 1800–1700 cm−1. Accordingly, in the present study,


Table 6Assignment of fundamental vibrations ofN-bromosuccinimide by normal mode analysis based on SQM force field calculations using selectively scaledB3LYP/6-311+G** force field

No. Symmetryspecies Cs

Observed fundamentals Calculated frequencies (cm−1) TED (%) among types ofinternal co-ordinatesd

FTIR FT-Raman Unscaled Scaleda Aib Ii

c

B3LYP/6-31G*

B3LYP/6-31G*

B3LYP/6-311+G**

1 A′ 3009 vw 3180 3017 3004 0.162 2.01 CH2 ass (96)2 A′ 2998 vw 3000 vw 3157 3005 2994 0.157 5.18 CH2 ass (98)3 A′ 2982 w 3133 2988 2985 0.092 26.26 CH2 ss (98)4 A′ 2912 vw 3055 2907 2910 0.171 2.46 CH2 ss (96)5 A′ 1759 m 1895 1766 1762 0.292 29.79 CO (86)6 A′ 1715 vs 1856 1712 1715 0.876 3.46 CO (88)7 A′ 1688 w 1817 1693 1690 0.212 2.32 CC (86)8 A′ 1514 vw 1630 1526 1520 0.184 5.11 CC (88)9 A′ 1468 ms 1589 1471 1469 0.176 6.20 CH2 sciss (84)

10 A′ 1412 w 1510 1414 1412 0.201 19.81 CH2 sciss (86)11 A′′ 1377 w 1463 1383 1381 0.179 0.69 CH2 wag (74)12 A′′ 1339 w 1418 1350 1346 0.116 13.72 CH2 wag (76)13 A′ 1312 vs 1385 1304 1317 0.813 0.87 CN (83), CC (10)14 A′ 1302 m 1366 1311 1298 0.234 33.10 CN (82), CC (11)15 A′′ 1250 ms 1309 1245 1253 0.623 0.73 CH2 twist (66)16 A′′ 1236 w 1283 1244 1230 0.226 17.42 CH2 twist (64)17 A′ 1212 vs 1260 1218 1216 0.743 6.23 CC (79)18 A′ 1022 w 1061 1030 1027 0.501 1.71 CH2 rock (69)19 A′ 1002 w 1044 1006 1005 0.478 8.26 CH2 rock (71)20 A′ 997 w 1030 999 997 0.291 2.91 R bend 1 (66), R bend 2 (21)21 A′ 890 w 928 883 894 0.182 3.08 R bend 2 (61), bNBr (10)22 A′ 815 ms 842 820 818 0.432 1.24 NBr (73)23 A′ 700 w 729 704 702 0.185 0.82 bCO (62), CC (19)24 A′ 606 ms 600 vs 624 600 609 0.251 64.76 bCO (63), CC (21)25 A′ 512 w 535 518 509 0.301 4.38 bNBr (64), CN (23)26 A′′ 486 s 490 s 507 497 493 0.262 53.48 �CO (53), CN (12)27 A′′ 425 vw 442 433 429 0.191 4.23 �CO (54), bCO (21)28 A′′ 280 s 301 290 286 0.462 2.34 �NBr (51)29 A′′ 255 vs 274 264 258 0.361 76.34 t Rtorsion1(56),�NBr (23),

�CO (21)30 A′′ 227 vw 231 w 239 238 234 0.143 5.08 t Rtorsion 2 (56)

a For optimised values of scale factors seeTable 4.b Relative absorption intensities normalized with highest peak absorbance equal to 1.0.c Relative Raman intensities calculated by Eq.(1) and normalised to 100.d For the notation used seeTable 3.Abbreviations used: ss: symmetric stretching; ass: antisymmetric stretching; b: bending; R: ring; t: torsion;�: out-of-plane

bending; vs: very strong; s: strong; ms: medium strong; m: medium; w: weak; vw: very weak.

the FTIR and Raman bands observed at 1768, 1701 and1753 cm−1, respectively, for succinimide and the Ramanband at 1759 cm−1 and FTIR band at 1715 cm−1 for N-bromosuccinimide have been assigned to CO stretchingvibrations. The in-plane and out-of-plane bending vibrationshave been also identified and presented inTables 5 and 6,respectively, for the title compounds.

5.3. N–Br vibrations

The attachment of bromine with the hetero atomgives rise to N–Br stretching and bending vibrations inN-bromosuccinimide. These vibrations are very difficult toidentify in the spectra, since mixing of bands of variousfunctional groups are possible in this region. However, basedon TED, the N–Br assignments were proposed. The FTIRbands at 815, 512 and 280 cm−1 have been assigned to N–Br

stretching, in-plane and out-of-plane bending vibrations,respectively.

5.4. C–N vibrations

The Raman bands appeared at 1348 and 1316 cm−1 in suc-cinimide, and 1312 and 1302 cm−1 in (IR and Raman bands)N-bromosuccinimide have been designated to C–N stretch-ing vibrations. For the title compounds a pure mode cannotbe expected for this vibration since, it falls in a complicatedregion of the vibrational spectrum[40,41].

6. Conclusion

Complete vibrational analysis of succinimide andN-bromosuccinimide were performed on the basis of DFT


calculations at the B3LYP/6-31G* and B3LYP/6-311+G**

levels of the theory. The role of methylene, carbonyl and othergroups in the vibrational frequencies of the title compoundswere discussed. The various modes of vibrations were unam-biguously assigned based on the results of the TED outputobtained from normal coordinate analysis. The assignment ofthe fundamentals is confirmed by the qualitative agreementbetween the calculated and observed band intensities (espe-cially with the large basis set) and polarization properties aswell and is believed to be unambiguous.

Acknowledgement

The authors are thankful to Sophisticated AnalyticalInstrumentation Facility (SAIF), IIT, Madras, Chennai forthe spectral measurements.

References

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