Electronic absorption spectrum of ortho-fluorophenol in vapor state

JODRNAL OF MOLK(:LJLAR SPIXTROSCOPY37, 486-493 (1971)

Electronic

Absorption

Spectrum Vapor G. N.

Departwent

of Physics,

R.

of o&o-Fluorophenol

in

State TRIPATHI

University

of Gorakhpur,

Gorakhpur,

India

The absorption spectrum of ostho-fluorophenol has been investigated in the near ultraviolet region. The (0,O) band has been identified at 2716.5 ii (36800 cm-l). Vibrational analysis for the rest of the bands has been proposed in terms of 6 ground-state (238, 437, 564, 763, 1023, and 1275 cm-‘) and 10 excited-state (109, 190, 293, 401, 507, 729, 935, 1052, 1128, and 1251 cm-l) fundnmental frequencies. A blue shift of 451 cm+ in the (0,O) band of o-fluorophenol, with respect t,o phenol, has been observed. Hertz and Bohlrausch (1) studied the Raman spectra of isomeric fluorophrnols. Vibr&ional analysis of the infrared and ultraviolet a.bsorption spectra of tbese molecules were attempted in our laboratory (W-4). The aim of this paper is to publish vibrational assignments for the ultraviolet spectrum of orthofluorophenol. An earlier work on this molecule states a fen- ground- and excited-state fundamentals (5). However, the present investigat,ion is more exhaustivr and reports a detailed analysis of the spectrum. EXPERIMENTAL

PROCEDURE

AND RESULTS

The chemical was procured from Columbia Organic Co., U.S.A. The spectrum was photographed on a Carl Zeiss Q24 spectrograph employing absorption path of 75 cm and varying the temperat,ure between 2%SO”C. Orthofluorophenol is liquid at room temperature and is very volatile. Only one drop of the liquid inserted into the absorption tube results in sufficient vapor pressure at’ room temperature to absorb the strong frequencies. With increase in the drops insidr the absorption tube and rise in the temperature, weak bands gather intensity, strong bands are broadened, and the region of ab:orpt.ion extends on both sides. The bands lie in the region 2815-2430 A. Xearly 350 bands have been measured. ;\lost) of t#he bands are sharp and have line-like struct’ure. A fen- of the b:Lnds on the lower-wavelength side are somewhat diffuse. The band positions in \\-Rv~’ numbers (corrected to vacuum), approximate intensities, and probable assignments of nearly 150 selected bands are tabulated in Table I. Table II prrsrnts a comparative study of the fundamentals of o-fluorophenol and fluoroanisolc (6). 486

SPECTRUM

OF ORTHO-FLUOROPHENOL

487

DISCUSSION

Phenol is a planar molecule in theoground state (7’). Vibrational assignment of its electronic spectrum near 2700 A has been investigated in detail by Bist, Brand, and Williams (8-10) ‘and Brand, Califano, and Williams (11). These authors infer a planar configuration for phenol in the corresponding electronic upper state on the basis of harmonic behavior of modes connectsed with out,-ofplane vibrations (10). Therefore, a planar configurat’ion is most probable in the ground and excited states of the o-fluorophenol molecule, which is obtained by replacing one of the hydrogen atoms, adjacent to the hydroxylic group, by a fluorine atom in phenol. As such, o-fluorophenol belongs to C, point group, the molecular plane being the only element of symmetry present in t’he molecule. The A,, -+ Beu transition of benzene is responsible for absorption in the 2600 d region in benzene derivatives. This is allowed as A’ -+ A’, under C, symmetry, the transition moment lying in the plane of the molecule. The group of bands, which includes the (0, 0) band, is the first prominent group on the longer wavelengt’h side of the spectral region. The Boltzmann factor weakens the frequencies on the higher wavelength side of the (0,O) band. Intense bands, which lie at wave-number separations 109, 190, 293, 401, 507, 729, 936, 1052 and 1251 cm-’ on t,he violet side of the (0, 0) band are assigned to 1-O and those on the red side at 235,437, 564, 763, 1023 and 1275 cm-’ to O-l kansitions. The prominent bands are accompanied by bands at 9, 35, 46 and 141 cm-’ on their red and at 30 and 69 cm? on t)heir violet side. The difference frequency 35 cm -’ is excited upt,o two quanta. The combination 401 cm-’ of the ground-state frequency 437 cm-’ with excit*ed-state fundamental results into a difference frequency = 36 cm-’ (437 - 401 = 36 cm-‘). However, if a combination of t.he ground-state vibrat.ion 437 cm-’ is attempted with the excit.ed-stat.e vibration 293 cm-’ the difference frequency 141 cm-’ is explained. -I appear almost with equal int’ensity in t,he specThe frequencies 35 and 141 cm brum which support,s these assignments. The difference frequency 46 cm-’ occurs as 2 X 46 with a little anhnrmonicity. It can be assigned as l-l transition of 238 cm-’ wit,11 its counterpart of 190 cm1 in the exthe ground-state vibration cited &ate. Higher intensity of the frequency 46 cm-’ compared t)o 35 cm-’ supportIs this view. The frequency differences on t’he violet side of the (0, 0) band are assigned as l-l’ t,ransit,ions (Table I). A The excited-state frequency 109 cm-’ has been chosen as a fundament,al. fundamental of this magnitude has also been observed in o-fluoroanisole. The next fundament,al assigned in the upper state is 190 cm-l. It) very probably represents the counterpart of the ground-stat)e fundament~al 235 cm . The nonplanar bending mode of t,he substituent group has been assigned a value 244 cm-’ in t,he ground state of phenol (9). The C--F out-of-plane binding vibration has also been assigned in this neighborhood in fluorobenzene (12). Therefore, the fundamentals 235 and 190 cm-’ identified in the ground and excited electronic states,

488

TRIPATHI

respectively, of o-fluorophenol can be ascribed to either of the two substituent groups in this molecule. However, the close similarity in the electronic spectrum of o-fluorophenol and o-fluoroanisole and occurrence of the fundamentals of nearly the same magnitudes (G.S., 228; E.S., 186) in o-fluoroanisole leads one to relate these vibrations to C-F out-of-plane bending mode in the lower and upper electronic states. The prominent bands at 37093, 37210 and 37307 cm-’ represent the excited Table I. Band frequencies and assignments for o-fluorophenol Y

Intensity temperature 60°C

35525 35635 35743 35777 35797 35896 35990 36001 36028 36037 36099 36126 36144 36190 3622.5 36256 36307 36317 36328 36354 36363 36463 36472 36482 36516 36554 36562 366 11

36621 36650 36659 36204 36715 36727 36744 36754

40°C

Assignment

-1275 -1165 -1057 -1023 -1003 -904 -310 -799 -772 -763 -701 -674 -656 -610 -575 -564 -493 -483 -472

O-1275 O-1023-141 o-1023-35 O-1023 o-564-437,0-763-238 O-763-141 O-76346 0 -763-35 O-763-9 O-763 o-564-141 o-437-238,0-763+190 O-763+109 O-56446 O-564-9 O-564 043746-g 043746 0-2x238,0-763+293 o-437-35 0437-9 0437 O-238-141 0-2x14146,0437+109 O-2x141-35 0-2x141,0-23846 o-437-190,0-238-9 O-238 O-141-46-9 O-14146 o-141-35 o-141-9 o-141,0-437+293, O-3x46 O-2x46 O-46-35 O-2x35 O-566+507,0-+6-9 o-239+190

25'C

1 0 1

2 2 4' 4 6 8

AU

; : 0' 1 : 5 2 2 2 : 3" 2 5 8 10 4 4 2

4

1:

3

9 12

2 2

1

: 5

-446 -437 -377 -328 -318 -284 -246 -238 -201-189 -178 -150 -141 -96 -85 -73 -56 -46

SPECTRUM

OF ORTHO-FLUOROPHENOL Table I (continued)

u

60°C 36765 36786 36793 36800 36913 36823 36530 368.54 36862 36869 36900 36909 36940 36951 36980 36990 37051 37060 37088 37098

37122 37201 3?262 37277 37287 37298 37.307 37376 37385 37418 37449 37457 37452 37497 37508 37521 37529 37560 3?579 37590 37601

AU

Intensity temperature 40°C

25'C 3 7 8 10

2 3 4 7 8 9 10 15 2

2

: 6 8 2 6 8

I. 9 3 9 10 E z 4 z 10

20 4" 4 2

Assigment

4

-35 -14 -7 -0 +13 +23 +30 +54 +62 +69 +100 +109 +141 +151 +180 +190 +251 +260 +288 +293 +322 +401 +462 +47? +487 +498 +507 +.576 +585 +618 +649 +657 +682 +697 +7oa +721 +729 +760 +779 +790 +z301

O-763+729,3-437+401 0-2x9 o-9 (O,O) band 0+30-2x9 0+30-g 0+30,0+109-35-46 0+293-238 0+109-46 0+507-437,o+109-35 0+100-9 0+109 0+190-'36 o+lYo-35 o+l~so-9' 0+190 Oc293-46 0+293-35 0+293-g

0+293 0+293+30 0+401 0+407-46 0+,507-35 0+729-238 0+507-g o+507 0+729-141-9 0+729-141,0+2x293

0+.507+109 O+-729 -46 -35 0+729-2x35 0+729-46 0+729-35,0+507+190 0+729-2x9 0+729 -9 0+729 0+729+30 0+935-141-g

0+935-141 0+507+293

489

TRIPATHI Table

Y

Intensity temperature 60°C

37628 37635 37647 37675 37688 37697 37718 37,735 37747 37766 37806 37816 37842 37852 37878 379 13 37928 37983 3?993

35006 38016 38029 38038 38051 38071 38120 38158 38173 38208 38827 38243 38257 3828.5 35367 38453 38468 38490 385OQ 34514 38554 38570 38595

40°C 8 5 5 5 6 : 13 6 2 4

(continued) AU

Assignment

25'C 5

7

1: 12

4

2 7

3

z 4 4 :: 8 4 7 4" ; 12 15 3 3 8 12 3 3 3 4 4 4

I

6

5

~828 +835 +847 +F?7s +888 +897 +91x +935 +947 +966 +1006 +1016 +1042 +1052 +1073 +1113 +1128 +1183 +1193 +1206 +12X6 +1229 +1238 +1251 +X71 +1320 +1358 +X375 +1408 +1427 +1443 +1457 +1455 +1567 +1653 +1668 +1690 +1700 +1714 +17.54 Cl770 +1798

0+729+109 0+935-46-35 0+3x293 0+9X-46 0+935-35

0+729+190 0+935 0+9x5+30

0+1052-46 0+1052-35 0+1052-9

0+1052 0+1052+30 0+1251-141 0+935+190,0+1128 0+1251-2x35 0+125146-9 0+1251-46 n+i251-35 0+935+293 0+729+507

0+1251 0+2x729-14146 0+2f729-141 0+1251+109 0+2x729-46-35 0+2x729-46 0+2x729-35 0+125+190,0+935+507 0+2x729 0+2x729+30

0+2x729+109 0+2x729+190 0+935+729 0+935+729+30 0+2x729+293-46 0+2x729+293-35

0+2x729+293,3+1251+507 0+935+729+109,0+1052+729 0+2x935-16-35

SPECTRUM

491

OF ORTHO-FLUOROPHENOL

Table I (continued) Intensity temperature

Y

60°C

40’C

38628 38661 38778 38793 38872 38987 39198 39290 39391

25’C

2 4 4 3 5 1:

+1828 +1861 +1978 +1993 +2072 +2187 +2398 +2490 +2.591

2 2 71

3 3

39503 39603 39704 39926 40456 40635 40842 4 1167 4 l.378 41916 42089 42656

2

1L 1 1 position

0+2x935-46 0+2x729+401,0+2x935 0+1251+729 0+1052+935 0+729+1050 0+1251+935,0+3x729 0+2x729+935 0+1251+729+507,0+2x1251 0+2x729+1128 0+2x729-935+190 0+2x729+1251 0+3x935 0+4x729 0+3x729+935 0+.%?29 0+4x729+935 0+3X729+2x935 0+6x729 0+.5x729+935 0+7x729 0+6x729+935 0+8x729

+2703 +2803 +2904 +3126 +3656 +3835 +4042 +4367 +4578 +Fj116 +5289 +5856

3’ 3 2 2 2 1

ti - band

Assignment

AJJ

in

cm-l,

Au

TABLE CORRELATION OFTHEFUNDAMENTALS AND COMPARISON WITH o-fluorophenol _____.__ Ground state Ramana

445 dp,na? 580 p,s 760 p,s 1030 p,s 1255 dp , s a p polarized;

U"

238 437 564 763 1023 1275

-

shift from the (0,O) band.

II

OFO-FLUOROPHENOL~BSERVED INUVAND THE FUNDAMENTALS OF 0-FLUOROANISOLE

RAMAN,

o-fluoroanisole Excited state

Ground state

109 190 293) 401) 507 729 935 1052 1128 1251

149 228 454

100 186 433)

518 764 -

477) 729 914) 1023 1128 1237

dp = depolarized;

nl = medium;

Excited state

s = strorlg;

Modes of vibration

C-F out of plane bending Comp0nent.s of the 608e?, vibration of benzene Symmetric ring stretching vibrations C-H planar bending vibrat,ions C-F strekhing mode ? = correlation

doltbtfld.

TRIPATHI

492

These frequencies form state fundament~als 293,401 and 507 cm-l, respectively. combination bands with other fundamentals. In the ground state of the molecule, there are two intense bands at (0, 0) band separations 437 and 564 cm-l which c:m possibly be correlated with these vibrations. We relat’e bhese t’o 608 ego vbration of benzene which splits into two tJotally symmetric components t Gn, HI) under C, point> group and both are sensitive to substitution. The groundstate fundamerual 564 cm-l has been associated wit.h t,he excited-&ate fundabetween the two should result’ in a difference mental 507 cm-‘. A transition frequency of 57 cm’. A weak band has been measured at 56 cm-’ separation OII the red side of the (0, 0) band. It has not, been possible to decide which of 293 and 401 cm-’ should be assigned as the the two upper-state fundamentals counterpart of the ground-st,ate fundamental 4X cm-‘. Both are helpful in explaining FU separation. The 608 es, frequency drops by 14% in the rscited state of benzene. The percentage drop in the former is 33 % and in t’hr latter fundamental 437 cm’ with the s “b. Therefore, a correlat,ion of t,he ground-state excited-st.ate fundament,al 401 cm-’ appears t.o be a better choice. The intense band at 37529 cm-’ involves t,he excitation of upper-state fundato (0, 0) band in intensity. The group of mrrit~al 729 cm-l and is comparable bands which contains this band is similar t’o the group of bands containing (0, 0) band in appearance. This fundamental combines with all the prominent groundand excited-st,at,e frequencies and results in intense bands. It is excited upt’o S quanta. Most, of the bands on t,he short’er wavelength side of the absorption region are explained as combinat,ion of this frequency. It forms a series of bands \vhich can be represented as v = n X 729 + vf , where P is the frequency shift of a band from (0, 0), n is number of quantum jumps in the fundamental 729 -’ and ZJ~is some upper-state fundament,al. This series is complete upto 1~ = 2. Fith Vf = 935 cm-l, the series runs upto n = 5. A fundamental of exactly t)his magnitude has been observed in o-fluoroanisole spectrum. These observations lead one to assign the upper-state fundamental 729 cm-l to a totally symmetric ring vibration. The upper state fundamental 935 cm-’ is responsible for an intense band at’ 37735 cm-‘. The fundamental combines wit,h almost all the upperstate fundament,als and can be assigned to a totsally symmetric vibration. The probable ground-state fundamentals which can be correlated with the excitedstate vibrat,ions 729 and 935 cm-l are 763 and 1023 cm-‘, respect,ively. These frequencies appear prominent,ly in Raman speet.rum and are polarized. Bist, Brand, and Williams (9, 10) assign two totally symmekic ring vibrations at. 992.3 and 823.2 cm-’ in the ultraviolet ground st,ate of phenol. Steele and Lippincott (12) have ident,ified the corresponding fundamentals at 100s and SO6 cm-’ in the ir spectrum of fluorobenzene vapour. The lower fundamental is sensitive to substitution and is therefore phenol. Therefore, ground-state counterparts

expect’ed to fall down considerably in o-fluorofundamentals 1023 and 763 cm-’ and t’heir

935 and 729 cm-‘, respectively,

in the excited state of o-fluorophenol

SPECTRUM

OF ORTHO-FLUOROPHENOL

493

are assigned as the totally symmetric ring breathing vibrations, following Steele and Lippincott (12) and Tripathi (3). The C-H planar bending vibrations lie between 950-1200 cm-’ in the upper state in benzene derivatives (4, 6, 8). The upper state fundamentals 1052 and 1128 cm-’ have been assigned to these modes. The band at 37051 cm-’ involves an excited-stat,e fundamental 1251 cm-‘. It corresponds to the frequency 1280 cm-’ (Tripathi, unpublished data). An extremely weak band in the uv ground state at 1275 cm-’ has also been observed. The 1310 bzu vibration of benzene is expected to appear in this neighbourhood in o-fluorophenol. However, in most of the fluorinated benzenes a frequency of this order has been attributed to the C-F stretching mode (13). The doublet separation of 9 cm-’ has been observed in the bands of o-fluorophenol. Small doublet separations of this order have been reported in the prominent bands of most of the fluorinat.ed benzenes. Sponer and Rao (25), have pointed out the possibility of assigning it as the rotational structure of the bands. The (0,O) band of phenol has been measured at 36348.7 cm-’ (14). The (0,O) band of o-fluorophenol is shifted to violet by 451 cm-’ with respect to this band. The high electronegativity of the fluorine atom is held responsible for this anomalous shift. ACKNOWLEDGMENT I am indebted to Professor D. Sharma, Gorakhpur, for his kind encouragement. RECEIVED:

January

Head of the Physics

Department,

University

of

6, 1970 REFERENCES

1. %. 3. 4. 6. 6. 7, 8. 9. 10. II. 12. I$. 14.

E. HERTZ AND K. W. F. KOHLRAUSCH, Mh. Chem. 76, 249 (1947). L. N. TRIPATHI, Ph.D. thesis, Gorakhpur University, Gorakhpur, India, 1966. G. N. R. TRIPATHI, Ind. J. Pure Appl. Phys. 7, 517 (1969). G. N. R. TRIPATHI, Ind. J. Pure Appl. Phys. 8, 157 (1970). S. K. TIWARI, Nature 260, 1202 (1963). L. N. TRIPATHI, Ind. J. Pure Appl. Phys. 7, 357 (1969). T. KOJIMA, J. Phys. Sot. Jap. 16,284 (1960). H. D. BIST, J. C. D. BR.~ND, AND D. R. WILLIAMS, J. Mol. Spectrosc. al,76 (1966). H. 1). BIST, J. C. D. BRAND, AND D. R. WILLI.%MS, J. Mol. Spectrosc. 24,402 (1967). H. D. BIST, J. C. D. BRAND, AND D. R. WILLIAMS, J. Mol. Spectrosc. 26, 413 (1967). J. C. D. BRAND, S. CALIFANO, AND D. R. WILLIAMS, J. Mol. Spectrosc. 26, 398 (1968). D. STEELE AND LIPPINCOTT, J. Mol. Spectrosc. 6, 238 (1961). H. SPONER AND K. N. R.40, Can. J. Phys. 36,332 (1957). J. CHRISTOFFERSEN, J. M. HOLLAS, AND G. H. KIRBY, Proc. Roy. Sot. A 307, 97 (1968)