Spectrochimica Acta Part A 71(2008)1128–1133
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Spectrochimica Acta Part A:Molecular and
Biomolecular
Spectroscopy
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s a
a
Vibrational spectra and structure of isopropylbenzene
A.I.Fishman a ,∗,A.I.Noskov a ,A.
B.Remizov b ,D.V.Chachkov b
a Kazan State University,Kremlevskaya Street 18,Kazan 420008,Russia
b
Kazan State Technological University,K.Marx Street 68,Kazan 420015,Russia
a r t i c l e i n f o Article history:
Received 7March 2008Accepted 7March 2008
a b s t r a c t
Infrared spectra (4000–400cm −1)and Raman spectra (1700–40cm −1)of the liquid and two crystalline solids of isopropylbenzene (cumene)and isopropylbenzene-d 12have been recorded.The spectra indicate that in the liquid and crystalline solids isopropylbenzene exists in planar conformation only (C H bond is in the plane of the benzene ring).An assignment of the obrved band wave numbers both isopropyl-benzene and isopropylbenzene-d 12is discusd by comparison with normal mode wave numbers and IR intensities calculated from ab initio 6-31G (d)force fields.
©2008Elvier B.V.All rights rerved.
1.Introduction
Molecular structure and conformational mobility of isopropyl-benzene (cumene)have been challenged in veral studies.Various experimental and theoretical methods have been applied,yet the data obtained so far do not allow one to interpret the conforma-tional behavior of this molecule unequivocally.
An electron diffraction study of a ries of isopropylbenzenes in the gas pha [1]has shown that the molecules of this class occur in two stable conformations.The C–H bond can be in the plane of the benzene ring (the planar or p -conformation,Fig.1a)and at some angle along the C(sp 2)–C(sp 3)bond,in particular,it can be per-pendicular to the benzene ring (the orthogonal or o -conformation,Fig.1b).Spectra of cumene and other alkylbenzenes,obtained with the help of the Time of Flight Mass Spectroscopy (TOFMS)tech-nique were investigated in [2].In the spectrum of cumene,only one inten line was obrved in the region of 37668.5cm −1,which was interpreted as the prence of a single p -conformation.A number of authors [3,4]conducted theoretical simulations of isopropylben-zene.Ab initio calculations using a Hartree–Fock lf-coordinated field with an STO-3G basis [3]have shown that p -conformation is preferable,with the global minimum on the potential curve cor-responding to it.The calculated barrier to internal rotation was found to be of 14.5kJ/mol.In Ref.[4]using a 6-31G basis t and optimizing the conformation at ϕ=90◦,it has been obtained that the energy of o -conformer is 13.4kJ/mol higher than that of the p -conformer.
∗Corresponding author at:Department of Physics,Kazan State University,Krem-levskaya street 18,420008Kazan,Russia.
E-mail address:**********(A.I.Fishman).To our best knowledge,there is only a single study of confor-m
ational stability in cumene by means of infrared spectroscopy [5].Main attention there was given to the analysis of the spec-tra between 1140and 1170cm −1,where IR spectrum of the liquid exhibits two absorption bands:1148cm −1and 1160cm −1.The authors claimed that some of the bands “freeze-out”upon crystal-lization.Relying on this fact,they concluded that there is a dynamic equilibrium of the two cumene conformations in the liquid pha.Unfortunately,no sufficient experimental evidence (either spectra or tables)is given in the paper to prove the conformer’s disappear-ance upon crystallization.The provided survey spectra of the liquid and crystal are insufficient for drawing any conclusion regarding possible conformational inhomogeneity in cumene.Thus,confor-mational stability of cumene in different phas,its vibrational spectra and their interpretation still reprent a challenge for spec-troscopists.
In this work we performed a new study of vibrational spectra of isopropylbenzene (Cum)and its fully deuterated analog (Cum-d 12).The goal of the study is to establish stable conformations of this compound in condend and crystalline phas.
绿茶的制作工艺2.Experimental
Fourier-transform infrared spectra were collected using a Bruker spectrometer Tenzor-27at a resoluti
on of 1cm −1and the num-ber of scans of 32.The liquid samples were droplets squeezed between KBr plates.Cryogenic studies were carried out using a standard cryostat.The temperature was registered by a platinum thermometer.The temperature was maintained with an accuracy of ±1K.
Raman spectra in the region 50–1700cm −1were collected using a modified spectrometer DFS-52equipped with a double mono-
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A.I.Fishman et al./Spectrochimica Acta Part A71(2008)1128–1133
1129
Fig.1.Schematic picture of isopropilbenzene:(a)planar and(b)orto. choromator.Helium–neon lar( =632.8nm,power40–50mW) operating in a continuous mode was ud as an excitation source. Radiation of helium–neon plasma was cut off with the help of an optical interferencefilter.Fine-tuning of the transmittance fre-quency was carried out by small inclinations of the opticalfilter. The spectral slit width did not exceed2cm−1.The integration time at each wavelength was2s.The sample was encapsulated in a glass capillary.A vacuum cryostat cooled with liquid nitrogen was ud for obtaining spectra at low temperatures.The temperature was measured using a copper-constantan thermocouple and main-tained with an accuracy of±1K.
3.Ab initio and normal-coordinate analysis
All calculations have been conducted using the Gaussian98pro-gram[6]with non-empirical method of functional density B3LYP [7]with the u of the basic t6-31G(d).Conformity of the found stationary points to the energy minima in all cas was proved by calculating the cond derivatives.The vibration forms and the powerfields were transformed from Cartesian to internal coordi-nates by using the program of Ref.[8].To eliminate the discrepancy between the calculations and the experimental data,scaling factors were employed in accordance with Ref.[9](e Table1).
Isopropylbenzene molecule has57normal vibrations.One of the molecular fragments,a mono-substituted benzene C6H5X,can be characterized by C2v symmetry.In its turn,the alkyl C3H7X fragment has C s symmetry.Conformations p and o both have C s symmetry,with the benzene ring laying in the symmetry plane for р-conformation,whilst the symmetry plane being perpendicular to the ring for o-conformation.Thus,the vibration symmetry of the alkyl fragment should be identical for the both conformers(Fig.1) whilst there are some differences for the vibration symmetry of the ring.
Tables2and3show the results of the normal-coordinate anal-ysis for cumene molecules and its deuterated counterpart.
Table1
Scaling factors for force constants of the different internal coordinate
Internal coordinate Scaling factor
Valence C C0.9207 C H(aromatic)0.9150 C H(aliphatic)0.8890
Deformation C C H0.9500 H C H0.9016 C C C 1.0144
Out of plane0.9760 Torsions0.95234.Results and discussion
We calculated frequencies and forms of normal vibrations for two conformations of Cum and Cum-d12and analyzed spectral regions in which the frequencies are noticeably different.
According to the calculations,the frequency and the form of a ries of normal vibrations are nsitive to molecular conformation (Tables2and3).When passing from p-to o-conformation,the most significant frequency changes are obrved in the following ranges (p/o):211/203,230/244,458/483,563/577,736/720,1055/1069, 1077/1091,1099/1124,1141/1154,1315/1323and1326/1336cm−1, respectively.Similar changes in the spectra of Cum-d12are obrved in the ranges(p/o):265/225,282/311,402/426,467/426, 641/629,868/884,827/841,734/728,641/629,467/426,402/373, 282/311and265/225cm−1,respectively.If cumene molecules indeed occur in veral non-identical conformations then a few bands in IR and Raman spectra would freeze out during sample crystallization.
Two crystal modifications of Cum and Cum-d12,exhibiting significantly different vibrational spectra,have been obtained in experiment.Both crystal forms were grown by cooling a liquidfilm of cumene between two KBr plates.When cooling the sample from 300to80K at the rate of∼30K/min,no crystallization has been obrved.During a subquent heating up to∼170K,the crystal form A(Table
s2and3,Fig.1b)was formed,which remained intact within the temperature range from80K up to the melting point (∼173K).
A different crystal form B(Tables2and3,Fig.2)was formed at a slower,∼0.5K/min,cooling of the liquid down to∼160K.This form was found to be metastable.During subquent heating,it transforms into the form A.
Bad on the obrved IR and Raman spectra of Cum and Cum-d12in the above specified conformational-nsitive regions it cannot be concluded that two stable conformers exist in the liquid pha.It is claimed in Ref.[5]that veral absorption bands from the range1148–1170cm−1“freeze-out”off the spec-trum upon
crystallization.Bad on their obrvations,the authors of[5]concluded that there is a dynamic equilibrium of two conformers and determined their enthalpy difference.According to our calculations(Table2),this spectral range contains two fundamental vibrations of p-conformer with the frequencies at Fig.2.IR spectra cumene of liquid(a),crystal A(b)and crystal B(c).
1130 A.I.Fishman et al./Spectrochimica Acta Part A71(2008)1128–1133
Table2
Experimental and calculated IR and Raman frequencies(cm−1)and PEDs for cumene
Infrared Raman Calculation PED
Liquid Crystal A Crystal B Liquid,Crystal A Crystal B Planar Orto
1668vw
1607sh1619sh
1603s1604m1603vs1607vw1610A’4.81609A’6.57o,p,60Qpp
1583vw1582vw1583vw1584vw1589A’11586A”1o,p,63Qpp+12p
1574sh
1564vw
1551vw1551vw
1527
1520
1493vs1494w1495s1492vw1508A’12.771511A”3.88o,p,40p+28Qpp
1487vw1483vw
1472m1469A’1.09p,24␣+24p+17Qpp
1464A’8.281476A’22.3o,p,83␣
1460A”3.59o,42p+26Qpp
1465s1465w1467m1473sh1458A”2.21457A”2.84o,p,81␣
1464m1464vw1443A’7.041455A’0.07o,p,84␣1452m1454vs
1452vs1445m1450s1447vw1448A”0.01441A”6.65o,p,88␣
1447sh1438sh1447sh1395vw
1395A’3.29o,47␣+42
大学班主任1383vs1380m1379vs1384vw1391A’2.161376A”3.68o,p,43␣+25 1364m1362sh
1363vs1361w1360m1371A”2.43p,42␣+26
1355m1352vw1363A’4.05p,32a+14pr
1336A”0.07o,52p+15pr
1324w1326vw1330w1320vw1326A’0.67p,76pr
1325sh
1322A”1.68o,37pr+24Q pp
1314w
1306w1308m1304vw1323A”3.08p,43p+19Q pp
1315A’26.4o,37a+24pr+12Q p 1303w,br1300vw1304m1300sh
1281m1281w1282vs1282vw1284A’4.88p,64Q pp
1283A”0.07o,66Q pp+13pr
1267vw
1253w1242vw
1213m1212w1214m1209m1216w1215w1207A’1.09p,31Q p
1199A’0.52o,28Q p+12
1203w12051202vw
1182sh
1181w1183w1180s1179w1182w1181vw1181A’0.051192A’0.58o,p,73p+18Q pp
1171w1166w
学习是一种信仰
1156w1157w1157vs1152w1159w1158w1159A’0.121161A”0.0o,p,70p+18Q pp
1149w1148w
1144w1144vw1144w1142vw1143w1147vw1154A’0.431141A’2.76o,p,36+12Q pp
1139w
1132w中国的对外援助
1114sh1114m1113m
1124A”0.05o,30Q+11Q pp+10p+101106m1105m1104vs1102w1105w1103w1099A’0.79p,50Q+16+10␥
1099sh
1082vs1084s1085vs1078w1084w1082w1091A’6.77p,30Q pp+25p
1077A’0.29o,40+15a
1052m1055m
1069A”2.98o,30Q+23Q pp
1049vs1048s1046vs1042m1055A’6.53p,24Q pp+22+22a 1032sh
1028vs1028m1027vs1025m1027w1026m1031A”7.41031A’6o,p,38Q pp+18p+18␥p 1020vw1017w
1001w1001w1001m997vs1001s999s998A’0.151000A’0.56o,p,50Q pp+28␥p
993sh
989vw990m
983w982w987sh985vw989vw989vw976A”0.15976A’0.08o,p,82+18972w971sh
965w969w950A”0.04p,20
949A”0.15o,30+25Q+22
949A”0.0o,52+10Q+10+10955w956vw952w952w940A”0.0p,40Q+40
940vw941vw942w
920m922m922m918vw917A’0.96912A”1.57o,p,72+17pr
917m
906m903s911m906vw908A”1.37905A’1.30o,p,72
891w891w890m893w892vw891w871A’0.92862A’1.39o,p,66Q
A.I.Fishman et al./Spectrochimica Acta Part A 71(2008)1128–1133
1131
Table 2(Continued )
Infrared Raman Calculation PED
Liquid Crystal A Crystal B Liquid,Crystal A Crystal B Planar Orto 883vw 882sh 842w 855w 856w 836w 843vw 844w 846A”0.0842A”0.0o,p,77+18851vw 853sh 761s
761s 769s 757vw 760vw 762vw 769A”22.1763A’24.4
o,p,49+17c 757sh
762w 741w 744w 743w 744w
744w
736A’1.18p,24Q p +14Q +12␥p 720A’5.94o,24Q p +14Q +12␥p 699s
698s 704s 707wv 704A”26.5
698A’21o,p,54+12+11c 701sh 620vw
619vw
617w 635w 619w 631A”0.01
632A”0.0o,p,62␥p +16p 577A’4.24o,22␥+11
587vw 594w 564w 563w
562w 565vw 564w
563vw 563A’0.18p,21␥p +18␥+12+10pr +10␥pa 536s 537s 550vw
541vw 543A”9.03
p,26␥pa +20+20␥+13
537vw
524sh?
526w
532sh
483A’2.33
o,19c +14Q p +13␥pr 457w
463vw
462w
458A’0.36
p,19c +14Q p +13␥pr 419A”0.47o,47␥+24␥pa 406A”0.0
o,39␥pr
409A”0.02
p,64+30
405A’0.5
o,51+15r +10␥p 322vw
322vw
356A’0.19
o,23␥+18Q p +14(CH 3)308vw
313vw
310vw
317A”0.32p,46␥+26306A’0.09p,45␥pr
261A’0.13
p,84(CH 3)
268A”0.0o,65␥pa +60(CH 3)244A’0.12
o,69(CH 3)230vw
230vw
230vw
230A”0.0p,98(CH 3)
211A’0.05
p,44␥pa +18(CH 3)+14␥203A”0.07o,70(CH 3)143A’0.59
o,35+26c
142w
151w 152m 127A”0.02
p,30c +29+24␥
90m 96m 80sh 62vs
68s 41A”0.0p,72p
63sh
Notation:(CH 3)-torsion CH 3group,p -torsion around C(Ph)-C(Pr),-C(Ph)-H out of plane,c -out of plane C(Ph)-C(Pr),␥-∠C(Ph)-C(Pr)-C(CH3),␥p -∠C(Ph)-C(Ph)-C(Ph),␥pa -∠C(Ph)-C(Ph)-C(Pr),␥p r-C(CH3)-C(Pr)-C(CH3),-∠C(Pr)-CH 3-H,a -∠C(Ph)-C(Pr)-H,p -∠C(Ph)-C(Ph)-H,pr -∠
C(CH 3)-C(Pr)-H,-ring puck,Q-C(Pr)-C(CH3),Qp-C(Ph)-C(Pr),Qpp-C(Ph)-C(Ph),␣-∠H-C-H in the CH 3.
Fig.3.IR spectra cumene of liquid (a),crystal A (b)and crystal B (c).
1144(36+12Q pp )and 1156cm −1(70p +18Q pp ),and also two similar modes of o -conformer (at 1141and 1161cm −1,respec-tively).However,in the IR spectrum of the liquid (Fig.3a)only two bands are obrved in that range.Both bands retain in the spectra of the crystals (Fig.3b and c).An additional band obrvable in the spectrum of crystal A could be attributed to Davydov splitting of the 1144cm −1mode in the crystalline pha.None of the bands of the liquid “freeze-out”upon crystalliza-tion.
It is of interest to analyze similar vibrations of Cum-d 12.Vibra-tional modes due to the angles in о-and р-conformations coincide at ∼986cm −1(Table 3).Another mode,which is preferably due to vibrations of angle p ,is shifted from 868cm −1in the planar conformation to 871cm −1in the orthogonal one.In the spec-trum of the liquid,a singlet absorption band is obrved in this range.
It has been noted above that the calculated frequencies of o -and p -conformations differ significantly in many spectral ranges.The analysis prented in Table 4shows that only p -conformation prents in the liquid.
1132 A.I.Fishman et al./Spectrochimica Acta Part A71(2008)1128–1133
Table3
Experimental and calculated IR and Raman frequencies(cm−1)and PEDs for cumene d12
Infrared Raman Calculation PED
Liquid Crystal A Crystal B Liquid,Planar Orto
1570vw1572vw1573vw1570w1571A’5.081571A’0.06o,p,70Q pp
1544vw1547A’0.21544A”4.81o,p,74Q pp
1383vw1381vw1382vw1385A’3.341383A’1.76o,p,44Q pp+26p+14Q p
1329vw1329vw1330vw1341A’0.911342A”7.29o,p,45Q pp+10p
1285vw1286vw1286vw1291A’0.571289A”1.39o,p,89Q pp
表示说的词1187A’7.29o,34Q p+17Q
1219vw1219vw
1214vw1212vw1212vw1213w1192A’3.67p,37Q p+8a
1199vw1199vw
1167vw1168vw1168vw1166w1152A’4.181164A”3.08o,p,56Q+16pr 1126vw1125vw
1107vw1106vw1108vw
1107vw1106vw1105vw1101A”0.581104A’1.77o,p,31␣+24+22Q 1061sh
1058m1055m1061m1054vw1053A’7.361058A’8.19o,p,90␣
1059sh
1051sh1051w1049w1050A’4.32p,34␣+32p
1053A”1.92o,70␣
1049A”0.631049A’5.95o,p,93␣
1045A”0.79o,44␣+14
1047A”1.94p,70␣+12pr
1043A’0.181042A”0.36o,p,96␣
1026A”0.0o,37p+18pr+11
1034vw1035vw1035vw1036vw1037A”0.0p,46␣+14+12pr
架空历史小说大全1025A’10.47o,14a+12pr
1022w1021w1021w1021m1024A’6.74p,14pr+12+11a+10␥p 1014vw1014vw1017sh
985vw985vw986vw985vw986A’0.55p,46
986A’0.12o,46+10pr
961vw960vw960vw960vs957A’0.17958A’0.0o,p,47Q pp+28␥p
951vw
906vw904vw905vw904vw902A”0.1899A”0.11o,p,33pr+22+18Q
884A’0.52o,28a+12c+10869w868w869w870s868A’0.1p,74p+11Q pp
871A’0.81o,69p
杉树寄生的功效与主治849vw847w847w851sh847A’4.8p,27p+23a
840vw844sh843sh840m842A’1.23843A”1.76o,p,61p
我喜欢这种感觉830sh830sh827A’1p,43p+12a+10
841A’2.28o,44p+20Q pp 824w824vw825vw825A”2.65834A”0.8o,p,86p
821vw
817sh816vw817A”0.29805A’0.0o,p,84
778vw779vw772A”0.0772A”0.0o,p,86
787vw790vw790vw768A’0.0p,49+14Q
778vw779vw
747A”0.73o,71b+18Q
753vw751vw752vw753w735A’2.34739A’3.87o,p,37+18Q+10
739vw741vw741vw734A”1.32p,42Q+20
728A’1.52o,28+20Q+10
673vw673w674w673s681A”0.16676A”0.68o,p,66+26pr
665A’0.69o,23␥p+16Q+15Q p+10p
656vw667A’1.97p,19␥p+13Q+12Q p+10
659A”0.0657A”0.0o,p,76+18
636vw642w645vw636w641A”1.17p,30+28+14c
642sh
629A’2.69o,28+27+18c
595vw595vw594vw595w605A’0.05605A”0.03o,p,58␥p+26p
550s554s558s550vw553A”14.38552A’14o,p,64+33
558sh555sh
493w495A’0.0p,22+13Q p+13␥p+12␥
495A’8.68o,29+14␥+11Q p+11459s464vs463vs467A”15.79p,16c+36+32
426A’5.14o,20c+16Q p+12
405vw402A’0.24p,14␥pr+16␥pa+12c+12Q+10pr
373A”0.39o,41␥+26␥pa
291vw356A”0.0353A”0.0o,p,86
344A’0.43o,44␥pr
311A’0.09o,32␥+14Q pp+12Q p
270vw282A”0.16p,40␥+33
265A’0.08p,50␥pr
225A”0.04o,44␥pa+25␥+15(CD3)
208A’0.08p,38CD3+28␥pa+10␥
184A”0.16o,70(CD3)+11␥pr
168A’0.0p,72(CD3)
166A”0.0155A”0.0o,p,96(CD3)
125A’0.41o,29+22c+17(CD3)+11␥
124s112A”0.26p,28c+27+24␥
36A”0.0p,72p
Notation:(CD3)-torsion CD3group,p-torsion around C(Ph)-C(Pr),-C(Ph)-D out of plane,c-out of
plane C(Ph)-C(Pr),␥-∠C(Ph)-C(Pr)-C(CD3),␥p-∠C(Ph)-C(Ph)-C(Ph),␥pa-∠C(Ph)-C(Ph)-C(Pr),␥pr-C(CD3)-C(Pr)-C(CD3),-∠C(Pr)-CD3-D,a-∠C(Ph)-C(Pr)-D,p-∠C(Ph)-C(Ph)-D,pr-∠C(CD3)-C(Pr)-D,-ring puck,Q-C(Pr)-C(CD3),Qp-C(Ph)-C(Pr),Qpp-C(Ph)-C(Ph),␣-∠D-C-D in the CD3.
