The infrared spectra of deuterated tetrahydrofurans

Spectr~himiea Aeta,1960,Yol. 16,pp. 459 to 106. Pergamon PressLtd. Printedin NorthernIreland

The in&wed spectra of deutemted te~~y~~~s* A. PALM and E. R. BISSELL Lawrence Radiation Laboratory, University of California Livormora, California, (Received 4 Juszuu~y 1960) Ab~tr~~-~~ infrared spectra of et series of deutemted tetr&hydrof~a~ have been studied in the gaseous, liquid and solid states in the region between 3200 and 350 cm-l. An attempt has

been made to assign the observed absorption bands to the normal modes of vibration, taking sdvantage of isotopic shifts and effects due to phase changes. The spectra of the liquid eornpo~~ show the usual displacements compared with those of the vapors; the solid-state spectra are intensifiedand reveal a sha~ening and splitting of the absorption bands. A number of peaks in the region between 1400 and 700 cm-l possess appreciable fine structure in the crystalEmestate. Introduction

the five-membered cyclic compounds, the fundamental vibrations of cyclopentane have been studied in most detail [l-4]. Spectroscopic studies of tetrahydrofuran (THF) in the liquid phase have been concerned with the Raman effect [5, 61 with the exception of those by TSCNAMLER and VOETTER [i’] wlxo also jn~estigated the infrared spectra. In the most recent absorption studies the salient features associated with the oxygen heterocyclic compound have been discussed f8, Qj+ The infrared spectra ofTlIP, the completely deuterated analog THE‘-&, and the tetradeuterated compounds THF-2:2:6:5-d,; THF-3:3:4:4-~~; and THF2:3:4:5-d, have been examined in the present research. The description 2:2:5:5-d, denotes a tetradeuterofuran in which the a-hydrogen atoms have been replaced by dcuterium atoms. The molecule THF-2:3:4:5:d, is deuterated at both the a- and &carbon atoms, Although a complete assignment of the observed vibrational bands cannot be carried out readily without a normal co-ordinate treatment, due to the strong interaction between the skeletal and the CH, group vibrations, the major components have been identified. AMONG

E~e~en~~ The infrared spectra were obtained by means of a Ferkin-Elmer model 21 donble-beam spectrometer equipped with NaCl and CsBr prism interchange units. Stray radiation was reduced in the C&r region with a roughened CaF, reflection filter mounted in place of the plane mirror in back of the exit slit of the instrument. A standard IO-cm Pyrex gas cell was provided with a vertical side arm, serving * This [I] [2] [3] [4] [5] [6] [7] IS] [3]

work was performed under the auspices of the U.S. Atomic Energy Commission.

J. E. KILPATRICBI, K. S.PITZER and R. SPITZER,J. Am. Chew. Sot. 69, 2483 (1947). F. A. M~LER end R. G. INSKEEP, J. Ohem. P&a. 18,1519 (1950). B. CURNUTTE, JR. and W. H. SHAFFER,J. MCI?.S~ec~oso. 1, 239 (1957). H. TSCRAMLIER and H. VOE~ER, Molaatsh. Chena.Sa, 835,1228 (1952). K. W. F. Komxu~sorr and A. W. REITZ, 2. p&&k. Cham. B 45,249 (1940). H. L-R, F. LAWI%, J. GOWEAU and W. RODEWALD, Z. ~~~~Q~~c~. 5, 34 (1950). H. Tscaa~aa~ea md H. VOE-R, Mona&h. Chem. 88, 302 (1952). G. M. B-ROW and S. SE~LXS, J. Am. Chem Lsoc. Xi,1175 (1953). 0. D. SHXLEVE, M. R. HEETHER, H. B. K~G’ELT and D. SWERN, Ati. C&epn.%%,277 (1951). 459

as a ~iq~d-simple reservoir. This was immersed in a small Dewax in order to vary the vapox pressure as required by the intensity of the absorpt~ou bands, For the liquid spectra, specially designed @I- and 0*5-mm sealed cells of the type described by BBUEQEL [lo] were employed. They offer the advantages inherent in both sealed and den~ountabIe eehs. The solid samples were examined as thin films in a modified low-temperat~e transgression cell [I If using liquid nitrogen as refrigerant; a brass frame served as structural support and for reproducible alignment and attachment to the spectrometer. The gas was transferred to the cell and allowed to condense on to a salt disk. This operation was performed quickly to prevent diEusion of the vapor on to the coId upper portion of the cell, The sample hofder was s~gbt~y rotated towards the spigot to assure au even deposition of the crystalline films. This tec~ique resulted in slight scattering below 1800 em-f. The probable accuracy of the speetral data is estimated to be about f2cm-1 below 1000 cm-l, and &5 cm-r and f fOom_l in the 2000 cm-l and 3000 cm-1 regions, respectively. The hrfrared spectra of the compounds in the vapor states are reproduced in Fig. 1. The indicated perceutage transmission represents merely a qua~tative relation within a given speotrum; sample pressures varied between 20 and 160 mm Kg as indicated. The vapor spectrum of THF has not been studied before, but the liquid and solution spectra have been reported previously [7-91. The weak bands that appear in the CH stretching region of the completely deuterated THF are due to au estimated 10 per oeut impurity of mouodenteratod THF. The l~q~d-state spectra of THF and of its deutera~d analogs are not reproduced, since onty minor differenoes have been observed between the vapor and the liquid states. In contrast, prominent changes occur in the solid, as ~nstrated in Big. 2, The investigations of the CsBr region of the liquid-state spectra have been limited to the range 650-350 cm-I. The preparation of THF and its deuterated analogs has been described elsewhere [12]. Results and interpretation Tetrahydrofuran may be represented as shown; only the 2:3:4:5tetradeuterated molecule can exhibit cis-trots isomerism. One hydrogen atom on each of the four carbon atoms is replaced by a denter~um atom. A total of six isomers is possible, but the method of preparation renders the three depicted here most probable:

For our purpose of analysis, a planar skeletal configuration is assumed since the distortion from planarity is slight [7]. Therefore THF, along with THF2:2:5:5-d, and THF-3:3:4:4-d,, assumes a C,, symmetry for which the selection fi;] W. BXWJJKXEL, ~~~~~~~~~ fllj flz]

E. Ii, E. R.

~ACWEB BISSELG

k &ie ~~~~~~~~~~~~~ p. 1%. ~t%inkopff, .33&Wadt and D. %‘. ~OmTrC%, J. ~?@?s. ~&8. %tB,296 jl%%@f. and i?% Fnwm, L7,Org. Ohm. 28,2366 @KS},

460

fL951).

The infrared spectra of deuterated tetrahyd~furans

EO

2600 2400 2xX1 2000 IBOOI600 I400 1200 DO0 800 WAVE NUMBER

INem-

and dauterated ~t~y~f~~ Fig. 1. Infrared absorption spectra of ~tr~y~~~ in the vapor state. (1) !FHF; (2) THB’-d,; (3) T@F-2:2:5:5-do; (4) THF-3:3:4:4-d,; and (5) THF-2:3:4:5-d,.

461

IO0

50

I08

50

C

1 I 1 I I I I I Z&XI 2800 2600 2400 2200 xxxf Ia00 1600 t4OC WAVENUM%ERIN cm-’

~g. 2. Infrared absorption spectra of t&mhydrofuran and deutemted tetrahydrofurmm ju the &id state as thin fk~a. (13 TIED?; (2) THF-d,; (3) THF-2:2:5:5&; (4) THF3:3:4:4-d,; and (6) !I%F-2:3:4:5-d,.

462

The infrared spectra of deuterated tetrahydrofurans

rules are summarized in Table 1. The most probable configuration of the 2:3:4:5analog is that of the C, point group, although the symmetry groups C, or C, are not excluded. Table 1. Symmetry species and internal ooSordinates of tetrahydrofuran,

based on a planar configuration

= lOu, f 7a, + 9b, + 7b, rring = ha, + la, + 3b, -t- 16, GH, = 6u, + 6a, f 66, + 6b, r a1 = ZCH stretch + ZCH, bend + ZCH, wag rc,,

r =z = 2CH stretch + ZCH, twist + 2CH, rock = 2CH stretch + 2CH, bend + 2CH, wag % = 2CH stretch + 2CH, twist + ZCH, rock %

The wave-numbers of the band centers of the five molecules examined are listed in Table 2. The slight variation among the isotopic species in the region of the CH stretching vibrations may be due to the fact that in the case of the THF-3:3:4:4-d, molecule the CH vibrations involve the @-carbon atoms, while in the THF-2:2:5:5-d, they are associated with the cc-carbon atoms which are adjacent to the C-O group. The @-C-H bonds should be less affected by this group, but the difference in wave numbers may not be significant as it is within the order of magnitude of the experimental error. The isotopic shifts of the CH stretching (2976 and 2964 cm-i) and CH, bending (1458 cm-l) modes are most prominent, whereas the absorption maxima ascribed to the skeletal modes (1177, 1076 and 912 cm-l) are displaced to a lesser extent. Fig. 3 presents a comparison between the spectra of the deuterated tetrahydrofurans in the vapor state and those in the liquid and solid states. The small frequency shifts accompanying this phase ohange are to be expected for nonpolar or slightly polar bonds. As may be seen from Figs. 1 and 2, the solid-state spectra exhibit more intense absorption peaks than the spectra of the vapors. This is a recognized phenonlenon and has been the subject of a recent review [13]. It is particularly striking in the region of the combination bands which, though rather faint in the vapor- and liquid-phase spectra, become more pronounoed in the spectra of the solids. In Table 3 the Reman [7] and infrared spectra of liquid THF and of its deuterated analog are compared with the infrared spectra of the solid compounds. The approximate assignments listed in the last column have been deduced on the basis of isotopic displacements and effects accompanying the transition from the gas to the solid phase. They are in essential agreement with those suggested by TSCHAMLER and VOETTER [7]. The CH stretching and CH, bending modes can be identified with reasonable certainty. Roth types of vibrations exhibit isotopic shifts tallying closely with the theoretical values. The distinctions between the CH, wagging, twist and rocking modes are more difficult to delineate because they are less localized and cannot These CH, vibrations also undergo always be described in a simple manner. isotopic shifts upon deuterium substitution, but their relative amounts may not be [13]C. A. Cou~so~,Spectrochim.

hta

14,183(1968).

A. PALM sxd E. R. BISSELL

3000

2600

2200

f8UU

1500

I300

IIW

900 I

700cm I

of the same order of magnitude. The C-C as well as the C-O vibrations are not expected to show depreciable isotopic displacements and can therefore be assigned on the basis of their speotral invariance. A comparison with the spectral data of eyclopentane and its dederated analog [2] confirms this interpretation” In the spectrum of gaseous THE’, the bands at 1366 and 1238 cm-l show a distinct ~Q~-st~cture, while the peak identified with the sym~~etri~ skeletal stretching mode appears to have a rotational contour also.

The infrared apsctrs. of deuteretad

btrahydrofurans

Table 2. Infrrtfed spectra of deuterctted t%trahydro~~~ vapor at.&%between 3000 and 650 cm-l THF 2976 vs* 2964 vs 2847 vs 2679 m

2001 m 1458 6

THF-d,

2232 vs 2222 vs 2094 vs

THF-2:2:5:5-d4

THF-3:3:4:4-c&

2962 vs

2977 vs

2881 vs

2855 vs 2614 m 2236 EI

2208 2103 1975 1469

8 vs w s

1299 1244 1192 1144 1091 1045

w w w,sh s V8 vs

1366 m 1238 s 1177 s 1076 vs

1156 vs 1106 vs 1058 vs

654 vs

846 s 743 s

1239 m 1185 w,sh 1135 s,sh 1083 vs 937 vs

912 8 821 w

2142 s 2013 w 1460 w 1373 m 1364 s i 1355 m

874 m 817 s 731 w 600 474 400 389

vst w 8 8

755 s 639 vs 494 w 424 w

in the THF-2:3:4:5-c& -__--_ 2962 vs 2869 vs 2689 w 2198 s,sh 2143 vs 1460 w 1311 rn 1268 m 1257 m 1070 vs 968 m 926 vs 916 vs i 905 vs 865 m 846 m 735 m 724 m 669 vs 400 9 388 B

* The estimltted intensities are given by w, weak, m, medium, s, strong, vs, very strong, sh, shoulder and bd, broad. t Liquid-stat& absorption bands below 650 cm-l.

Splitting of absorption maxima observed in the solid-state spectrum is particularly prominent in the case of the bands associated with the skeletal vibrations. The frequency ~spl~cexnents compared with the gas-phase spectra are the usual ones; the bands ascribed to the CH stretching modes show a distinct red shift, in contrast to those of the CH, bending vibrations which undergo a slight blue shift. The displacements of the remaining vibrations are less pronounced in magnitude as well as direction. Most noteworthy is the fact that the as-type vibrations of the isolated molecules, forbidden in the infrared ape&rum but permitted in the Raman effect, appear in the absorption spectrum of the solid. Although X-ray studies of THF have not been reported in the literature, it is expeated that there will be more than one molecule per unit cell so that the site symmetry will be lower than that of the free molecule. As a, result, in the crystalline state, the a2 vibrations will be active in the infrared as well as in the Raman spectrum. The bands at 2938 and 964 cm-l ma+y then be ascribed unequivoca;lly to a2 vibrations. The assignment of

A. PALM and E. R. BISSELL Table 3.

Raman and infrared spectral data and probable assignments

Liquid

Solid * Probable assignments_$

Infrared absorption bands (cm-l)

Ramant

THF

THF

THF-d,

-_ 2975 2938 2865

2977 vs

2226 vs

2861 vs

2133 s,sh

2717

2680 m 1972 m

2093 vs 1884~

1461 s

1099 vs

1486 1452

THF

IK V “: 2849 vs

1

1364 m 1333 w

1234 1174

1289 m 1234 m 1177 8

1162 vs

1104 1071 1028

1067 vs 1030 m,sh

1051 vs 749 m

964 913

908 vs

651 596 276 215

654 s

842 m

I

THF-ds

/

2694m _ 1935 w 1889 w 1723 w 1487 m 1466 m ( 1441s 1421 w,sh 1368 m 1339 w 1323 m I 1307 m 1241 s 1179 vs 1150 w 1108 w,sh 1058 vs 1043 vs 980 w,sh 954 s 9218 1908 s 891 s 871 s 838 vs 725 w 662

2236 w,sh 2202 s 2147 s 2111s 2085 s 1871 m 1795 w 1155 1100 1080 1136 998 983 962

CX stretch (a) CX stretch (a) CX stretch (s)

m vs m w m w w

CX, CX,

wag wag

CX, twist ring stretch (a) CX, rock

1035 m,bd 758 s

CX,

8 s w 8

rock

ring stretch (s)

!

I

bend bend bend

CX, wag CX, wag ring stretch (a)

899 s 1171 m 927 w

704 832 810 745

c c x c CX, CX, CX,

CH, ring ring ring

rock i-p bend o-p bend o-p bend

* The samples were held at about - 180°C. t Reference [7]. $ Aside from the abbreviations which are self-explanatory, (s) and (s) refer to antisymmetric and symmetric, respectively; i-p and o-p denote in-plane and out-of-plane; C indicates a combination band.

the 1104 cm-l band is probably correct in spite of its weak absorption in the infrared spectrum. The band at 1486 cm-l cannot be identified unambiguously; although it follows the pattern of the a2 vibrations, it should be assigned preferably to a CH, bending mode. A parallel analysis cannot be carried out as readily for the deuterated molecule THF-d, since its Raman spectrum is not available, but the observed isotopic displacements make a partial assignment possible. 466