- Email: [email protected]
Association of tert-butyl alcohol: a matrix infrared study
of Molecutar Structure, 40 (1977) 13-23 8 Eisevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Zoumal
ASSOU,iTION STUDY
JOUKO
OF TERT-BUTYL
ALCOHOL:
A MATRIX
INFRARED
KORPPI-TOMMOLA
Deptu tment of Physical Hetsirlki I7 (Finland) (Received
12 November
Chemistry.
University
of Helsinki.
Meritullinkatu
1. SF-001 70
1976)
ARSTRACT Infrared spectra of (CH,),COH, (CH,),COD, (CD,),COH and (CD,),COD have been studied in the liquid and solid states, in CCI, and CS, solutions, and in argon, krypton and nitrogen matrices. The Raman spectra of the solids and of CCI, and H,O solutions have been recorded. The matrixinfraredresultsshow that in the matrices tert-butyl alcohol has two well-defined dimer associates, both of which seem to have an open-chain structure. A network is proposed for the higher associates. The relative strengths of the protium and deuterium bonds are discussed in terms of the monomer hydroxylstretching frequencies and the Au shifts. Association characteristics of the most important fundamental bands in several spectra are given. INTRODUCTION
The association of 2methyl-2-propanol (tert-butyl alcohol, TB) has been investigated by thermodynamic [ 1,2] , infrared [ 3,4] and NMR studies. Various methods of determination give values from 18 to 25 kJ mol-’ for the hydrogen bond energy of TB [5-S]. However, the models for the structure of the ticiates vary from work to work. In two NMR studies [7,8], both of whichincluded vapour pressure measurements, the experimental results have been interpreted in terms of trimer associates. In one NMR work [5] and in a near-infrared work [3], dimer and high polymer associates have been proposed for W-t-butyl alcohol. Tert-butoxides have been shown [9] to form tetramer aggregates in strongly solvating solutions. Whether the associates are open-chain or cyclic seems to be open to discussion. In ibis study, the infrared spectra of tert-butyl alcohols in argon and nitrogen matrices have been recorded at several M/A (matrix to absorber) ratios from M/A = 500 to M/A = 50. After a careful assignment of the vibrational
bands of isolated TBs [ 10 1, the influence
of the association
on
the whole vibrational spectra was examined. The spectra of the condensed phases (solution, liquid and solid) of TB recorded at normal temperatures were then compared wlth.the matrix results.
14
EXPERIMENTAL Reference 10 gives details of the origin and purification of the tert-butyl alcoholsand matrix’gases and of the synthesis of the.OHdeuter&d compounds. The infrared spectra were recorded on a Perkin-Elmer 621 spectrometer. The wavelength scale was calibrated with the aid of atmospheric water and carbon dioxide. The matrix isolation system has been described.earlier [l-l]. The temperature of ..theCsI depositionwindow was kept.af about 9 K during recording of the spectra. During the depositions the temperature was about 14-15 K for argon and 12-13 K for nitrogen matrices. To ensure the di.ffusion of the molecules through the matrices, the temperature was raised to 30-40 K for 6-12 minutes and then cooled back to 9 K. Throughout the experiments, the deposition rate was about 5 mm01 of the gas mixture per hour. The pressures of the alcohol and the matrix gas were measured with a mercury manometer. The Raman spectra of the compounds were recorded on a Jarrell-Ash 25-305 spectrometer with the 488 run line of an argon ion-laser as the exciting’line, in most cases using 90" excitation. In all measurements, the spectral slit width &as kept constant at about 2.5 cm”‘. RESULTS
AND
DISCUSSION
The following discussion is based on material published in ref. 10, and on additional material given in Tables l-3 of the present paper. Tables of the infrared and Raman frequencies of tert-butyl alcohols in the liquid and solid states and in solutions are’available from the author on request.
The hydroxyl
stretching
band and the association
models
The u (OH) association.band of TB and of its deuterated isomers con+stS of two sharp, well-resolved peaks’and of a broader component in argon matrix infrared spectra (see Figs. 1 and 2).,The frequencies of the:peaks correspond to the frequencies of the dimerbands found by. seve+.authors in the gaseous [12]. and CC&&&ion spectra [3,4] ;>The dimergeik df. lower hequency in the argon matrix ‘spectrais split’ in.the nitrdgen-matrix. spectra; an additional dimer band. appears in the nitrogen..matrixq&tra,~at 3425 and 2535 cm-’ fbr.thd OH and.OD alcohols, respectively.~.The latke~ band is also found when,&& &gonmatr&r & dopedxwftb ~g~:i&%ting a complex of the dix&r.tith nitrogen;.-Accord.inglji;&he~&xxi&ion of tert butyl alcohol in the &tricB.&ay be des&b& ti t&&‘of’d.imers and polymers, the latter of which is understood as asingle group without apecification of the structures. Since.trimer bands were not identified with certainty either in the gaseous or in the mat&r spectta; the assumption that trimers are the main associates of TBs [7,8] seems utiiounded
16 TABLE
1
Hydroxyl and CO stretching phases (cm-‘)
Species second
Monomer
fundamental
Vapour
PoIymer Polymer Monomer Monomer
3643.1 3548 3631
1330.1
Polymer Polvmer Dimer Monomer Monomer Polpner Polymer Polymer Monomer
Ar matrix -
site monomer
Secondsite monomer FreeOHofassociate Dimer Dimer Dimer Dimer Polymer Polymer Polymer
bands for OH tert-butyl
3626.8
3622.3 -
3635 3627.8 -
Solid
-
-
3619.0 3615 3506 -
3500.8 3473.9 3466.2 3424.5b 3275 3243
3360 -
1407 2380 1328.2
1402 137s 1343.7 1334.2
1400=
1210 1200 1183 1144.6 1139.5
1206 1191 1176 1162.4 1148.0
1202a 1188 -
226.9
216.5
222.5
778 696 628 324.0 310.1
in several
CC&solution
3506.3 3478.8 3420 3340 3246
-
Monomer
N, matrix
alcohols
1328
3590 -
3320 3270 1402 -
MOH) ._
i
6(OH)
1139
640a
NOHI
Species
Vapour
Ar matrix
N, matrix
Liquid
Second sitemonomer Monomer Fe OHofaeociate Dima
3643.6 3543
3628.0 3606;7
3634 3627.2
3623
LX&r
-
3479.8
DimDimPolsrmer Polymer Polymer Polvm=
-
3501.1 3471.3 3464.0 3425.2b
-
-
3410 3345 3256 3125
3275 3245 -
;360 -
POIpmLE Polymer Dimer Monomer tioaomer
1337.9
1385 1370 1336.6
1407 1392 1372 1356.4 1338.9
-
6
1x59 1167 1144 1133.6
1160 ii68 1146.7 1142.0
-
tic01
221.7 214.5
322.3 309.3
-
Pqjymer PO&IZIet
-
bfOZlOm#lE xoaomer
1135.9
Monomer Monomer
221.0
-‘From liquid i.r.spectra.bN,-induced.
Solid
v
r
TABLE
2
Hydroxyl
fundamental bands for OH-deuterated
tert-butyl alcohols in several phti
(Cm-’ )
(CH,I,COD Species
Vapour
Ar matrix
N, matrix
Liquid
Solid
hlonomer Second site monomer
2688.6 -
2676.5 2673.6 2591.3
2676.3 2586.2
2674.5 2669 -
-
2571.6 2530
2566.7 2552.7 2635.2b -
-
-
2480 2413 -
2475 2414 2660
2490 --
2471 -2421 11
950
950
Free OH of associate Dimer
-
Dimer Dimer Dimer Polymer
-
Polymer Polymer sire monomer Second
-2692.6
Pokner Monomer
-
947
945 886.4
-
PolYmer Monomer
- 877.7
875.5 960
963 883.2
-
Polymer Monomer Monomer
-
171.1” 167.6’
479 244.2 234.3
-
Species
Vapour
Second site monomer Monomer Free OH of associate Dimer Dimer Dimer Pobrmer Polymer Polymer
-
Polymer Polymer Dimer Monomer Monomer Monomer Monomer Monomer
-
165=
Ar matrix
N, matrix
2688.8 -
2678.2
2681 2677.8 -
-
2592.5 2573.1 -
-
2621 2480 2415
__ 894.4
-
960 940 904 898 892.6
I66.7*
E:f
aCakulated with the aid of the isotopic shift
-
v(OD).
480 icOD,
Liquid
Solid
.-
-
2588.2 2565.8 2533.5b -
2673.6 2669 -
-.
-
2470 2412
2500 -
2461 2438
961 946 928 -
NODI
11
-_ -
905.5 902.0
-
242.8 233.0
-
‘,
944
-
946
-
)
HdR)
ratio in nitrogennizitrixSPech. bN,-ifidriced.
Which dimer structures are most probable for tert-butyl alcohoPTh@ minimum energy conformation of tert-butyl alcohol is that with the hydroxyl group in a dihedral plane of the carbon skeleton [lo]. CJW&C dyers of water and methanol are known both from experiments [13--153 fid theory [lS, 171 to be energetically unfavoured. We think cyclic structures unlikely for TBs too, and hence explain our spectroscopic results in terms of
17 3 Hydroxyl stretching shifts AV for several associates of methanol, ethanol and tert-butyl alcohol (argon matrix frequencies, cm-‘) Species
Me
Dker C’ Dimer C: Polymer
126.2 139.2 370
aThe frequencies are fr
+ 89.1 99.5
1
EtOHa
EtOD*
r-
-311,
TBd,
TB-d,,
123.4 130.4 386
89.0 94.6 265
L,ZV.~ 148.0 382
05.2 104.9 264
121.3 148.4 378
85.7 105.1 263
13. TB = (CH,),
3.
open-chain dimers only. Two structures are now proposed as models for the TB dlmers. One of them possesses a plane of symmetry and is thus of C, symmetry (Fig. 3a). The other occurs in two spectroscopically-equivalent configurations, with the hydroxyl bond of the proton donor oriented in the direction of one of the acceptor oxygen lone pair orbitals. This second dimer model has no symmetry and is labelled a C, dimer (Fig. 3b). It is obtained from the C, dimer by rotating the proton donor molecule about 60” from the symmetry plane of the C, dimer. Both the proposed models allow of immediate extension to open higher associates: the addition extending the chain structure, attacks the oxygen atom of the end proton donor (Fig. 3). In the C, model, the resulting aggregates have all hydrogen bonds in a plane, whereas in the C, model a three-dimensional network is obtained. The lower frequency dimer band in all TBs (at about 3479 and 2572 cm-’ for the OH and OD alcohols, respectively) is assigned to the hydrogen bonded v(OH) of the C, dimer. This band shows splitting in the nitrogen matrix. The splitting may be due to the influence of the nitrogen molecules on the degenerate energy levels of the bonded v(OH) vibrations of the two equivalent configurations-of&he C, dimer. Then, according to our model, the higher . .._a#. . e.___ -I I .. -VW frequency dimer band of the TT ’ ’ * and .OD alcohols, respectively) the C;dir&r. Hence, the C, dim energy and a more stable structure than the CS dimer. The above assignmc supports the localiied-orbital picture .of an O-H bond approaching a lone pi of electrons,,rather than between the two lone pairs in the acceptor CO(Fig; 3).,,A smilarresult has been obtained for linear water dimers by means ab initio calculations fl6]. --: .-Tert-Butyl alcohol is also slightly associated in the gas phase 1121, the c b&d:beinigat 3640 cm-’ m.the,infmred spectra of both TB and TB-& ( In ccl, sohltio
b Y
2700
2500 (cm-l)
Association bands of tert-butyl alcohol in the infrared spectra: (a) vapour, pressures from the top i. 20 and 40 mm Hg; (b) argon matrices, M/A ratios from the top 1000 and 200, whilst the third curve was recorded after the diffusion of the molecules in the M/A = 200 matrix; (ci) Ccl; solution. 0.1 and 0.5 mol I-‘; (d) liquid.
Fig. 1 v(OH) 1 m cuvette,
Fig. 2 v(OD) Association bands of (CH,),COD in.the matrix infrared spectra: (a) argon matrices; (b) nitrogen matrices, M/A ratios from the top 2000,200 and 60 in both figures. The third curve from the top in both figures was recorded after the diffusion of the molecules in the MIA = 200 matrix.
band contour irrespective of the position of the analyzer:for the liquid TBs and for several Ccl, solutions (Fig. 4). The asymmetrk of. theassociation bands of TB and TB-d lo are &I@’to:cdmbination bands [lo] - It seems obvious that a variety of tertlbutyl al&ho1 asscxiates are present in the liquid and in solution, in contrast to the specific struchW?s found in matrices. As the depolarization measurements gave no evidence of “~metric vibrations” of the hydrogen-bonded ~pxies. it is believed that the 8saociat.e~. the dimers included, have open structures in solution and in the liquid State.
Isotopic
effect in hydrogen
bonding
The isotopic ratios Av*/Az+ were found to be markedly larger than the ratios Y: /VP and VP /VPfor the T&j. where AU = v. -v, and the indexes 0 and
19
Fig. 3 Projection of the mbdels for tert-butyj akuhol dimets. carbons. Methyl hydrogens are not seen ia the figwe.
Numbered
atoms are
(cn?) Fig.-4 @(OH) -cAtion JJqiid; (b) 75%.(~/w)
bands of tert-butg) ~JCOJIOJin tbe &man CCJ. solution: (cl 0.5% CCJ, solution.
spectra:
(a)
neat
i Merto the monomer and the ith associate,respectPely. Has thisany physicalmeaning?Let us examine the open-chainstructuresproposed above for the C, and C, dimers of tert-butyl alcohol. Let F and F’ be the force constsn~ oITthe nonbonded and the bo droxyl bonds, respectively; let us further assumethat the hydra brationsdo not affect the zx~rznal modes of the proton acceptor e. The latter assumptionseems
20
quite a reasonable approximation, since the hydroxyl stretching fre_quenci%_ are far from the other vibrational frequencies of the associates. The bonded hydrogen atom is ccksidered as a point mass vibrating between two strings (force.constants f and F’) attached to opposite walls. If the hydrogen‘bond energy originates mainly from the hy&oxyl bond of the proton donor .(i.e., when f = 0 then F F F’) we obtain for the hydrogen bond stretching force constant of the ith dimer fi
n* = 4s2c2m, [vO
+‘1
= 47r*c*/.IToTy~
(1)
where c~TOT is the reduced mass of two associate molecules, y0 the hydrogen bond stretching frequency, and mP the proton mass. Exact analogues are obtained for the deuterium-bonded system. Using argon matrix frequencies, the V~ frequencies have been calculated for methanol, ethanol and tert-butyl alcohol dimers (Table 4). The order of magnitude of the force constants calculated fromeqn. I is comparable to the force constants reported for water dimers [18]. Hydrogen bond stretching frequencies are predicted amazingly well, bearing in mind the crude approximations made. In fact, the V, values calculated for the deuterium bonds are higher than those for the protium bonds, as hti been found experimentally [ll] .for methanol and ethanol in matrices; (the.assignment~of the 213 cm-’ band of MeOD to v, in ref. 11 can equally .well be changed, and the 230 cm-’ band used instead, because of the overlap of the OD torsion band in the region). The relative strengths of the protium and deuterium bonds are calculated via the identity
x cc++vp)/(vo”
+ vi”)]
(2)
where yH and yD refer to the hydroxyl stretching frequencies _of the nondeuterated and deuterated alcohols, respectively. Applying eqn. .l to eqn:2, z&i. rearranging, we get
TABLE 4 u0 Frequencies (cm-‘) for methanol, ethanol and,tert-butyl model for the dimers and the force constants from eon. 1 Species Dimer C s Dimer C,
MeOH*
MeODa
EtOHa
alcohol using&e
diatomic
EtOD’
TB
‘J??-di
TB-;d,
TB&,
obs. talc. ohs.
223.
230
3l3
214
-
-
-
-.
238 223
240 236
196 213
200 214
162 -..
156 -
I.?? -
147 -
CalC.
250
253
202
207
169
l-71-
159
162
aTbe observtid.freqCeocies are from-ref. 13;
~-
where (~2 + uF)/(Y,D +-VP) has been replaced by v~/v~. For the justification of the last step see Tables 1 and 2. Thevalue of the ratio 4vH/4v,., can exceed a, since the denomin&or of eqn. 3 contains a frequency difference. Equation 3 shows that the relative positions of the monomer and dimer bands on the frequency scale.depend on the relative strengths of the deuterium and protium bonds. From eqn. 3, it can be seen that the stronger the deuterium bond, the huger becomes 4v, as compared with Avn. The force constant ratios f,.,/fH have been calculated for methanol, ethanol and tert-butyl alcohol u&ng.argon matrix frequencies. The results are given in Table 5. The deuterium bond seems to be stronger than the protium bond for all alcohols studied, the relative strength of the deuterium bond for ethanol being the largest. The deuteration ‘of the TB skeleton does not seem to influence the relative strength. If the dimer band of TB-d, had been found 3 cm-‘ towards the higher wavenumber end of the argon matrix spectra, then, according to eqn. 3, equally strong protium and deuterium bonds would have been obtained for TB and TR-cZ,. On studying the hetcroassociation of CH30H and TB with various proton acceptors, Rao [ 19 J found slightly higher complexation energies for the association of the OHdeuteratcd alcohols than for the nondeuterated alcohols. The result was interpreted in terms of strongerdeuterium bonds, an interpretation which paralfels that presented here. The relation 4v(OH) = 1.414 Au(OD) obtained for several 1:l complexes of TB by Rao [19] is completely consistent with the self-association shift ratios found in the matrix spectra for the TBS. The values reported in ref. 20 for the protium and deuterium bonding of methanol are doubtful, since relative intensities have been used for the estimation of the bond strengths. Association chamcteristics of some fundamental bands of TBs The hydroxyl-bending bands for tert-butyl alcohols in the matrix spectra consist of a-monomer and a polymer band (Fig. 5), the latter being some 40-80 cm-! higher in frequency than the former (Tables 1 and 2). If some other fuzdamexital band occurs in the frequency region of the polymer band, considerable intensity enhancement of the association band follows (Figs. 5c andd). The fiquencies of the monomer 6 (OH) bands are almost the same invapour, matrix and’CC14 solution spectra. TABLE
5
f&fH values calculated from eqn. 3 for methanol. (argoti matrix frequenhs uskd) Swcies
MeOH
EtOH
TiS
TBd,
Dim&r. Cj DiarWC;,
1.041 kO55
lx63 &OS8
1.043 i.045
1.042 1.045
ethanol and tert-butyl alcohol
dimers
I
1
1500
1
1
1300
1 #Q
1 1 ’ 1600 800
(cm-l) Fig. 5 6 (OH) Association bands of tert-butyl &ohols in the infrared spectra of argon (a) (CD,),COH, M/A = 150; (b) the (a) matrix after warming to 35 K and. cooling back to 9 K; (c) (CH,),COD, M/A = 200; and (d) the (c) matrix after warming to 35 K and cooling back to 9 K. matrices:
The r(OH) associationbands were.detectedin the liquid state spectra,the frequenciesbeing about 640 and 480 cm-’ for,TB and TB-di, respectively. As is usualwith alcohols, no-hydroxyl torsion bandswerefound in-the Ramanspectraof the liquids, althoughthe CH,torsion.bands were.observed. For ah isotopic tert-butylalcohols the 6 (CCO) band had an association band about 15-20 cm-’ on the high-frequency-sideof the.monomer band. The 915 cm-’ band of TB has associationbands on both sides of the major peak in ah matrixspectra.The detection of associationbands on both sides leadsone to assignthe 915 Cm-* band to two fundamentalvibrations[lO’J In the solid state Raman spectra,the band comprisesfour components, two of which are associationbands. CONCLUSION
The associationbandsof the alcohols may be classifiedinto two groups: those which arisefrom direct influenceof the hydrogenbond on the vibrationalmodes of the bonds invoIvedin association.and those which arise
23
from overall molecular interactions with the vibrational modes. Polymer bands of the former type are normally shifted from 20 to 400 cm-’ from the monomer band, depending on the alcohol and the fundamental concerned. The association bands due to molecular interactions are usually within a few
cm-* of the,monomer band, and are normally found as broadenings of the vibrational bands concerned. In order to distinguish between the bands of the two groups we call the bands of the former group alcoholic association bands and those of the latter group molecular association bands. If the association models proposed for tert-butyl alcohol prove acceptable for other alcohols, the hydroxyl stretching, bending and torsion and the CC0 bendingbands should be alcoholic association bands. The intensity of these bands clearly increases as the M/A ratio decreases or the alcohol molecules are made to diffuse through the matrix. The molecular association bands can be resolved only in the matrix spectra, whereas the alcoholic association bands can be detected also in solution, liquid and solid state spectra and sometimes in the vapour spectra. ACKNOWLEDGEMENTS
The author gratefully acknowledges financial support from the Emil Aaltonen Foundation and the Science Research Council of Finland. The advice of Associate Professor Juhani Murto and the helpful discussions of Associate Professor Antti Kivinen during thr course of this work have been much appreciated. REFERENCES 1 2. 3 4
S. Otin, M. Gracia and C. G. Lcsa, J. Chim. Phys. Physicochim. Biol. 71 (1973) 637. J. KenttZmaa, E. Tommila and M. Martti, Ann. Acad. Sci. Fenn. Ser. A 2, (1959) 93. U. Liddel and E. D. Becker, Spectrochim. Acts, 10 (1957) 70. A. Kivinen, J. Murto, J. Korppi-Tommola and R. Kuopio. Acta Chem. Stand., 26 (1972) 904. 5 J.-C. Davis, Jr.. K. S. Pitzer and C. N. R. Rae, J. Phys. Chem., 64 (1960) 1744. 6 M. Mercher. J. Rouviere, B. Brun and J. Salvinien, J. Chim. Phys. Physicochim. Biol., 69 (1972) -721. 7 R E_ Tucker and E D. Becker, J. Phys Chem.. 77 (1973) 1783. 8 W. Storek and H. Kriegsmann, Ber. Bunsenges, Phys. Chem., 72 (1968) 706. 9 P. Schmidt, L. Lochmann and B. Schneider, J. Mol. Struct., 9 (1971) 403. 10 J. Korppi-Tommola, Spectrochim. Acta. in press. 11 J. Murto, A. Kivinen, K.~Edelmann and E. Hassinen, Spectrochim. Acta. Part A, 31 (1975) 479. 12 A. J. Barnes..H. E. Hallam and D. Jones, Proc. R. Sot. London Ser. A, 335 (1973) 97. 13 A. J. Barnesand H.E. HaEarn. Trans. Faraday Snc.. 66 (1970) 1920.1932. 14 A. J..Tursi and E. R. Nixon, J. Chem. Phys., 52 (1970) 1521. 15 LJ. Bellamy and R J. Pace, Spectrochim. Acta, 22 (1966) 525. 1’6 P. &.-Kbllman~and’L. C. Allen, Cbem; Rev., 72 (1972).283. 17 H. Mbrita and. S;-Nagakura;Theor. Chim; Acts. 27 (1972) 325. 2.8 G. C. Piientel and .A L McClellani The Hydrogen Bond, W. H. Freeman, San Fiancixa~aridLondon, 1960, p. 135. 19 C. Ni R. Rao,J. Chem; Sot: Faraday Tmns. I,4 (1975) 980. 20 0. D.Ijonner, J. Chem. Thermodyn.. 2 (1970) 577.
Zoumal
ASSOU,iTION STUDY
JOUKO
OF TERT-BUTYL
ALCOHOL:
A MATRIX
INFRARED
KORPPI-TOMMOLA
Deptu tment of Physical Hetsirlki I7 (Finland) (Received
12 November
Chemistry.
University
of Helsinki.
Meritullinkatu
1. SF-001 70
1976)
ARSTRACT Infrared spectra of (CH,),COH, (CH,),COD, (CD,),COH and (CD,),COD have been studied in the liquid and solid states, in CCI, and CS, solutions, and in argon, krypton and nitrogen matrices. The Raman spectra of the solids and of CCI, and H,O solutions have been recorded. The matrixinfraredresultsshow that in the matrices tert-butyl alcohol has two well-defined dimer associates, both of which seem to have an open-chain structure. A network is proposed for the higher associates. The relative strengths of the protium and deuterium bonds are discussed in terms of the monomer hydroxylstretching frequencies and the Au shifts. Association characteristics of the most important fundamental bands in several spectra are given. INTRODUCTION
The association of 2methyl-2-propanol (tert-butyl alcohol, TB) has been investigated by thermodynamic [ 1,2] , infrared [ 3,4] and NMR studies. Various methods of determination give values from 18 to 25 kJ mol-’ for the hydrogen bond energy of TB [5-S]. However, the models for the structure of the ticiates vary from work to work. In two NMR studies [7,8], both of whichincluded vapour pressure measurements, the experimental results have been interpreted in terms of trimer associates. In one NMR work [5] and in a near-infrared work [3], dimer and high polymer associates have been proposed for W-t-butyl alcohol. Tert-butoxides have been shown [9] to form tetramer aggregates in strongly solvating solutions. Whether the associates are open-chain or cyclic seems to be open to discussion. In ibis study, the infrared spectra of tert-butyl alcohols in argon and nitrogen matrices have been recorded at several M/A (matrix to absorber) ratios from M/A = 500 to M/A = 50. After a careful assignment of the vibrational
bands of isolated TBs [ 10 1, the influence
of the association
on
the whole vibrational spectra was examined. The spectra of the condensed phases (solution, liquid and solid) of TB recorded at normal temperatures were then compared wlth.the matrix results.
14
EXPERIMENTAL Reference 10 gives details of the origin and purification of the tert-butyl alcoholsand matrix’gases and of the synthesis of the.OHdeuter&d compounds. The infrared spectra were recorded on a Perkin-Elmer 621 spectrometer. The wavelength scale was calibrated with the aid of atmospheric water and carbon dioxide. The matrix isolation system has been described.earlier [l-l]. The temperature of ..theCsI depositionwindow was kept.af about 9 K during recording of the spectra. During the depositions the temperature was about 14-15 K for argon and 12-13 K for nitrogen matrices. To ensure the di.ffusion of the molecules through the matrices, the temperature was raised to 30-40 K for 6-12 minutes and then cooled back to 9 K. Throughout the experiments, the deposition rate was about 5 mm01 of the gas mixture per hour. The pressures of the alcohol and the matrix gas were measured with a mercury manometer. The Raman spectra of the compounds were recorded on a Jarrell-Ash 25-305 spectrometer with the 488 run line of an argon ion-laser as the exciting’line, in most cases using 90" excitation. In all measurements, the spectral slit width &as kept constant at about 2.5 cm”‘. RESULTS
AND
DISCUSSION
The following discussion is based on material published in ref. 10, and on additional material given in Tables l-3 of the present paper. Tables of the infrared and Raman frequencies of tert-butyl alcohols in the liquid and solid states and in solutions are’available from the author on request.
The hydroxyl
stretching
band and the association
models
The u (OH) association.band of TB and of its deuterated isomers con+stS of two sharp, well-resolved peaks’and of a broader component in argon matrix infrared spectra (see Figs. 1 and 2).,The frequencies of the:peaks correspond to the frequencies of the dimerbands found by. seve+.authors in the gaseous [12]. and CC&&&ion spectra [3,4] ;>The dimergeik df. lower hequency in the argon matrix ‘spectrais split’ in.the nitrdgen-matrix. spectra; an additional dimer band. appears in the nitrogen..matrixq&tra,~at 3425 and 2535 cm-’ fbr.thd OH and.OD alcohols, respectively.~.The latke~ band is also found when,&& &gonmatr&r & dopedxwftb ~g~:i&%ting a complex of the dix&r.tith nitrogen;.-Accord.inglji;&he~&xxi&ion of tert butyl alcohol in the &tricB.&ay be des&b& ti t&&‘of’d.imers and polymers, the latter of which is understood as asingle group without apecification of the structures. Since.trimer bands were not identified with certainty either in the gaseous or in the mat&r spectta; the assumption that trimers are the main associates of TBs [7,8] seems utiiounded
16 TABLE
1
Hydroxyl and CO stretching phases (cm-‘)
Species second
Monomer
fundamental
Vapour
PoIymer Polymer Monomer Monomer
3643.1 3548 3631
1330.1
Polymer Polvmer Dimer Monomer Monomer Polpner Polymer Polymer Monomer
Ar matrix -
site monomer
Secondsite monomer FreeOHofassociate Dimer Dimer Dimer Dimer Polymer Polymer Polymer
bands for OH tert-butyl
3626.8
3622.3 -
3635 3627.8 -
Solid
-
-
3619.0 3615 3506 -
3500.8 3473.9 3466.2 3424.5b 3275 3243
3360 -
1407 2380 1328.2
1402 137s 1343.7 1334.2
1400=
1210 1200 1183 1144.6 1139.5
1206 1191 1176 1162.4 1148.0
1202a 1188 -
226.9
216.5
222.5
778 696 628 324.0 310.1
in several
CC&solution
3506.3 3478.8 3420 3340 3246
-
Monomer
N, matrix
alcohols
1328
3590 -
3320 3270 1402 -
MOH) ._
i
6(OH)
1139
640a
NOHI
Species
Vapour
Ar matrix
N, matrix
Liquid
Second sitemonomer Monomer Fe OHofaeociate Dima
3643.6 3543
3628.0 3606;7
3634 3627.2
3623
LX&r
-
3479.8
DimDimPolsrmer Polymer Polymer Polvm=
-
3501.1 3471.3 3464.0 3425.2b
-
-
3410 3345 3256 3125
3275 3245 -
;360 -
POIpmLE Polymer Dimer Monomer tioaomer
1337.9
1385 1370 1336.6
1407 1392 1372 1356.4 1338.9
-
6
1x59 1167 1144 1133.6
1160 ii68 1146.7 1142.0
-
tic01
221.7 214.5
322.3 309.3
-
Pqjymer PO&IZIet
-
bfOZlOm#lE xoaomer
1135.9
Monomer Monomer
221.0
-‘From liquid i.r.spectra.bN,-induced.
Solid
v
r
TABLE
2
Hydroxyl
fundamental bands for OH-deuterated
tert-butyl alcohols in several phti
(Cm-’ )
(CH,I,COD Species
Vapour
Ar matrix
N, matrix
Liquid
Solid
hlonomer Second site monomer
2688.6 -
2676.5 2673.6 2591.3
2676.3 2586.2
2674.5 2669 -
-
2571.6 2530
2566.7 2552.7 2635.2b -
-
-
2480 2413 -
2475 2414 2660
2490 --
2471 -2421 11
950
950
Free OH of associate Dimer
-
Dimer Dimer Dimer Polymer
-
Polymer Polymer sire monomer Second
-2692.6
Pokner Monomer
-
947
945 886.4
-
PolYmer Monomer
- 877.7
875.5 960
963 883.2
-
Polymer Monomer Monomer
-
171.1” 167.6’
479 244.2 234.3
-
Species
Vapour
Second site monomer Monomer Free OH of associate Dimer Dimer Dimer Pobrmer Polymer Polymer
-
Polymer Polymer Dimer Monomer Monomer Monomer Monomer Monomer
-
165=
Ar matrix
N, matrix
2688.8 -
2678.2
2681 2677.8 -
-
2592.5 2573.1 -
-
2621 2480 2415
__ 894.4
-
960 940 904 898 892.6
I66.7*
E:f
aCakulated with the aid of the isotopic shift
-
v(OD).
480 icOD,
Liquid
Solid
.-
-
2588.2 2565.8 2533.5b -
2673.6 2669 -
-.
-
2470 2412
2500 -
2461 2438
961 946 928 -
NODI
11
-_ -
905.5 902.0
-
242.8 233.0
-
‘,
944
-
946
-
)
HdR)
ratio in nitrogennizitrixSPech. bN,-ifidriced.
Which dimer structures are most probable for tert-butyl alcohoPTh@ minimum energy conformation of tert-butyl alcohol is that with the hydroxyl group in a dihedral plane of the carbon skeleton [lo]. CJW&C dyers of water and methanol are known both from experiments [13--153 fid theory [lS, 171 to be energetically unfavoured. We think cyclic structures unlikely for TBs too, and hence explain our spectroscopic results in terms of
17 3 Hydroxyl stretching shifts AV for several associates of methanol, ethanol and tert-butyl alcohol (argon matrix frequencies, cm-‘) Species
Me
Dker C’ Dimer C: Polymer
126.2 139.2 370
aThe frequencies are fr
+ 89.1 99.5
1
EtOHa
EtOD*
r-
-311,
TBd,
TB-d,,
123.4 130.4 386
89.0 94.6 265
L,ZV.~ 148.0 382
05.2 104.9 264
121.3 148.4 378
85.7 105.1 263
13. TB = (CH,),
3.
open-chain dimers only. Two structures are now proposed as models for the TB dlmers. One of them possesses a plane of symmetry and is thus of C, symmetry (Fig. 3a). The other occurs in two spectroscopically-equivalent configurations, with the hydroxyl bond of the proton donor oriented in the direction of one of the acceptor oxygen lone pair orbitals. This second dimer model has no symmetry and is labelled a C, dimer (Fig. 3b). It is obtained from the C, dimer by rotating the proton donor molecule about 60” from the symmetry plane of the C, dimer. Both the proposed models allow of immediate extension to open higher associates: the addition extending the chain structure, attacks the oxygen atom of the end proton donor (Fig. 3). In the C, model, the resulting aggregates have all hydrogen bonds in a plane, whereas in the C, model a three-dimensional network is obtained. The lower frequency dimer band in all TBs (at about 3479 and 2572 cm-’ for the OH and OD alcohols, respectively) is assigned to the hydrogen bonded v(OH) of the C, dimer. This band shows splitting in the nitrogen matrix. The splitting may be due to the influence of the nitrogen molecules on the degenerate energy levels of the bonded v(OH) vibrations of the two equivalent configurations-of&he C, dimer. Then, according to our model, the higher . .._a#. . e.___ -I I .. -VW frequency dimer band of the TT ’ ’ * and .OD alcohols, respectively) the C;dir&r. Hence, the C, dim energy and a more stable structure than the CS dimer. The above assignmc supports the localiied-orbital picture .of an O-H bond approaching a lone pi of electrons,,rather than between the two lone pairs in the acceptor CO(Fig; 3).,,A smilarresult has been obtained for linear water dimers by means ab initio calculations fl6]. --: .-Tert-Butyl alcohol is also slightly associated in the gas phase 1121, the c b&d:beinigat 3640 cm-’ m.the,infmred spectra of both TB and TB-& ( In ccl, sohltio
b Y
2700
2500 (cm-l)
Association bands of tert-butyl alcohol in the infrared spectra: (a) vapour, pressures from the top i. 20 and 40 mm Hg; (b) argon matrices, M/A ratios from the top 1000 and 200, whilst the third curve was recorded after the diffusion of the molecules in the M/A = 200 matrix; (ci) Ccl; solution. 0.1 and 0.5 mol I-‘; (d) liquid.
Fig. 1 v(OH) 1 m cuvette,
Fig. 2 v(OD) Association bands of (CH,),COD in.the matrix infrared spectra: (a) argon matrices; (b) nitrogen matrices, M/A ratios from the top 2000,200 and 60 in both figures. The third curve from the top in both figures was recorded after the diffusion of the molecules in the MIA = 200 matrix.
band contour irrespective of the position of the analyzer:for the liquid TBs and for several Ccl, solutions (Fig. 4). The asymmetrk of. theassociation bands of TB and TB-d lo are &I@’to:cdmbination bands [lo] - It seems obvious that a variety of tertlbutyl al&ho1 asscxiates are present in the liquid and in solution, in contrast to the specific struchW?s found in matrices. As the depolarization measurements gave no evidence of “~metric vibrations” of the hydrogen-bonded ~pxies. it is believed that the 8saociat.e~. the dimers included, have open structures in solution and in the liquid State.
Isotopic
effect in hydrogen
bonding
The isotopic ratios Av*/Az+ were found to be markedly larger than the ratios Y: /VP and VP /VPfor the T&j. where AU = v. -v, and the indexes 0 and
19
Fig. 3 Projection of the mbdels for tert-butyj akuhol dimets. carbons. Methyl hydrogens are not seen ia the figwe.
Numbered
atoms are
(cn?) Fig.-4 @(OH) -cAtion JJqiid; (b) 75%.(~/w)
bands of tert-butg) ~JCOJIOJin tbe &man CCJ. solution: (cl 0.5% CCJ, solution.
spectra:
(a)
neat
i Merto the monomer and the ith associate,respectPely. Has thisany physicalmeaning?Let us examine the open-chainstructuresproposed above for the C, and C, dimers of tert-butyl alcohol. Let F and F’ be the force constsn~ oITthe nonbonded and the bo droxyl bonds, respectively; let us further assumethat the hydra brationsdo not affect the zx~rznal modes of the proton acceptor e. The latter assumptionseems
20
quite a reasonable approximation, since the hydroxyl stretching fre_quenci%_ are far from the other vibrational frequencies of the associates. The bonded hydrogen atom is ccksidered as a point mass vibrating between two strings (force.constants f and F’) attached to opposite walls. If the hydrogen‘bond energy originates mainly from the hy&oxyl bond of the proton donor .(i.e., when f = 0 then F F F’) we obtain for the hydrogen bond stretching force constant of the ith dimer fi
n* = 4s2c2m, [vO
+‘1
= 47r*c*/.IToTy~
(1)
where c~TOT is the reduced mass of two associate molecules, y0 the hydrogen bond stretching frequency, and mP the proton mass. Exact analogues are obtained for the deuterium-bonded system. Using argon matrix frequencies, the V~ frequencies have been calculated for methanol, ethanol and tert-butyl alcohol dimers (Table 4). The order of magnitude of the force constants calculated fromeqn. I is comparable to the force constants reported for water dimers [18]. Hydrogen bond stretching frequencies are predicted amazingly well, bearing in mind the crude approximations made. In fact, the V, values calculated for the deuterium bonds are higher than those for the protium bonds, as hti been found experimentally [ll] .for methanol and ethanol in matrices; (the.assignment~of the 213 cm-’ band of MeOD to v, in ref. 11 can equally .well be changed, and the 230 cm-’ band used instead, because of the overlap of the OD torsion band in the region). The relative strengths of the protium and deuterium bonds are calculated via the identity
x cc++vp)/(vo”
+ vi”)]
(2)
where yH and yD refer to the hydroxyl stretching frequencies _of the nondeuterated and deuterated alcohols, respectively. Applying eqn. .l to eqn:2, z&i. rearranging, we get
TABLE 4 u0 Frequencies (cm-‘) for methanol, ethanol and,tert-butyl model for the dimers and the force constants from eon. 1 Species Dimer C s Dimer C,
MeOH*
MeODa
EtOHa
alcohol using&e
diatomic
EtOD’
TB
‘J??-di
TB-;d,
TB&,
obs. talc. ohs.
223.
230
3l3
214
-
-
-
-.
238 223
240 236
196 213
200 214
162 -..
156 -
I.?? -
147 -
CalC.
250
253
202
207
169
l-71-
159
162
aTbe observtid.freqCeocies are from-ref. 13;
~-
where (~2 + uF)/(Y,D +-VP) has been replaced by v~/v~. For the justification of the last step see Tables 1 and 2. Thevalue of the ratio 4vH/4v,., can exceed a, since the denomin&or of eqn. 3 contains a frequency difference. Equation 3 shows that the relative positions of the monomer and dimer bands on the frequency scale.depend on the relative strengths of the deuterium and protium bonds. From eqn. 3, it can be seen that the stronger the deuterium bond, the huger becomes 4v, as compared with Avn. The force constant ratios f,.,/fH have been calculated for methanol, ethanol and tert-butyl alcohol u&ng.argon matrix frequencies. The results are given in Table 5. The deuterium bond seems to be stronger than the protium bond for all alcohols studied, the relative strength of the deuterium bond for ethanol being the largest. The deuteration ‘of the TB skeleton does not seem to influence the relative strength. If the dimer band of TB-d, had been found 3 cm-‘ towards the higher wavenumber end of the argon matrix spectra, then, according to eqn. 3, equally strong protium and deuterium bonds would have been obtained for TB and TR-cZ,. On studying the hetcroassociation of CH30H and TB with various proton acceptors, Rao [ 19 J found slightly higher complexation energies for the association of the OHdeuteratcd alcohols than for the nondeuterated alcohols. The result was interpreted in terms of strongerdeuterium bonds, an interpretation which paralfels that presented here. The relation 4v(OH) = 1.414 Au(OD) obtained for several 1:l complexes of TB by Rao [19] is completely consistent with the self-association shift ratios found in the matrix spectra for the TBS. The values reported in ref. 20 for the protium and deuterium bonding of methanol are doubtful, since relative intensities have been used for the estimation of the bond strengths. Association chamcteristics of some fundamental bands of TBs The hydroxyl-bending bands for tert-butyl alcohols in the matrix spectra consist of a-monomer and a polymer band (Fig. 5), the latter being some 40-80 cm-! higher in frequency than the former (Tables 1 and 2). If some other fuzdamexital band occurs in the frequency region of the polymer band, considerable intensity enhancement of the association band follows (Figs. 5c andd). The fiquencies of the monomer 6 (OH) bands are almost the same invapour, matrix and’CC14 solution spectra. TABLE
5
f&fH values calculated from eqn. 3 for methanol. (argoti matrix frequenhs uskd) Swcies
MeOH
EtOH
TiS
TBd,
Dim&r. Cj DiarWC;,
1.041 kO55
lx63 &OS8
1.043 i.045
1.042 1.045
ethanol and tert-butyl alcohol
dimers
I
1
1500
1
1
1300
1 #Q
1 1 ’ 1600 800
(cm-l) Fig. 5 6 (OH) Association bands of tert-butyl &ohols in the infrared spectra of argon (a) (CD,),COH, M/A = 150; (b) the (a) matrix after warming to 35 K and. cooling back to 9 K; (c) (CH,),COD, M/A = 200; and (d) the (c) matrix after warming to 35 K and cooling back to 9 K. matrices:
The r(OH) associationbands were.detectedin the liquid state spectra,the frequenciesbeing about 640 and 480 cm-’ for,TB and TB-di, respectively. As is usualwith alcohols, no-hydroxyl torsion bandswerefound in-the Ramanspectraof the liquids, althoughthe CH,torsion.bands were.observed. For ah isotopic tert-butylalcohols the 6 (CCO) band had an association band about 15-20 cm-’ on the high-frequency-sideof the.monomer band. The 915 cm-’ band of TB has associationbands on both sides of the major peak in ah matrixspectra.The detection of associationbands on both sides leadsone to assignthe 915 Cm-* band to two fundamentalvibrations[lO’J In the solid state Raman spectra,the band comprisesfour components, two of which are associationbands. CONCLUSION
The associationbandsof the alcohols may be classifiedinto two groups: those which arisefrom direct influenceof the hydrogenbond on the vibrationalmodes of the bonds invoIvedin association.and those which arise
23
from overall molecular interactions with the vibrational modes. Polymer bands of the former type are normally shifted from 20 to 400 cm-’ from the monomer band, depending on the alcohol and the fundamental concerned. The association bands due to molecular interactions are usually within a few
cm-* of the,monomer band, and are normally found as broadenings of the vibrational bands concerned. In order to distinguish between the bands of the two groups we call the bands of the former group alcoholic association bands and those of the latter group molecular association bands. If the association models proposed for tert-butyl alcohol prove acceptable for other alcohols, the hydroxyl stretching, bending and torsion and the CC0 bendingbands should be alcoholic association bands. The intensity of these bands clearly increases as the M/A ratio decreases or the alcohol molecules are made to diffuse through the matrix. The molecular association bands can be resolved only in the matrix spectra, whereas the alcoholic association bands can be detected also in solution, liquid and solid state spectra and sometimes in the vapour spectra. ACKNOWLEDGEMENTS
The author gratefully acknowledges financial support from the Emil Aaltonen Foundation and the Science Research Council of Finland. The advice of Associate Professor Juhani Murto and the helpful discussions of Associate Professor Antti Kivinen during thr course of this work have been much appreciated. REFERENCES 1 2. 3 4
S. Otin, M. Gracia and C. G. Lcsa, J. Chim. Phys. Physicochim. Biol. 71 (1973) 637. J. KenttZmaa, E. Tommila and M. Martti, Ann. Acad. Sci. Fenn. Ser. A 2, (1959) 93. U. Liddel and E. D. Becker, Spectrochim. Acts, 10 (1957) 70. A. Kivinen, J. Murto, J. Korppi-Tommola and R. Kuopio. Acta Chem. Stand., 26 (1972) 904. 5 J.-C. Davis, Jr.. K. S. Pitzer and C. N. R. Rae, J. Phys. Chem., 64 (1960) 1744. 6 M. Mercher. J. Rouviere, B. Brun and J. Salvinien, J. Chim. Phys. Physicochim. Biol., 69 (1972) -721. 7 R E_ Tucker and E D. Becker, J. Phys Chem.. 77 (1973) 1783. 8 W. Storek and H. Kriegsmann, Ber. Bunsenges, Phys. Chem., 72 (1968) 706. 9 P. Schmidt, L. Lochmann and B. Schneider, J. Mol. Struct., 9 (1971) 403. 10 J. Korppi-Tommola, Spectrochim. Acta. in press. 11 J. Murto, A. Kivinen, K.~Edelmann and E. Hassinen, Spectrochim. Acta. Part A, 31 (1975) 479. 12 A. J. Barnes..H. E. Hallam and D. Jones, Proc. R. Sot. London Ser. A, 335 (1973) 97. 13 A. J. Barnesand H.E. HaEarn. Trans. Faraday Snc.. 66 (1970) 1920.1932. 14 A. J..Tursi and E. R. Nixon, J. Chem. Phys., 52 (1970) 1521. 15 LJ. Bellamy and R J. Pace, Spectrochim. Acta, 22 (1966) 525. 1’6 P. &.-Kbllman~and’L. C. Allen, Cbem; Rev., 72 (1972).283. 17 H. Mbrita and. S;-Nagakura;Theor. Chim; Acts. 27 (1972) 325. 2.8 G. C. Piientel and .A L McClellani The Hydrogen Bond, W. H. Freeman, San Fiancixa~aridLondon, 1960, p. 135. 19 C. Ni R. Rao,J. Chem; Sot: Faraday Tmns. I,4 (1975) 980. 20 0. D.Ijonner, J. Chem. Thermodyn.. 2 (1970) 577.