Vibrational spectroscopy of hydroxy-heterobiaryls

Spectrochimica Acta Part A 54 (1998) 1291 – 1305

Vibrational spectroscopy of hydroxy-heterobiaryls I. Low frequency modes P. Borowicz a, O. Faurskov-Nielsen b, D.H. Christensen b, L. Adamowicz c, A. Les´ d, J. Waluk a,* b

a Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 -224 Warsaw, Poland Department of Chemistry, Uni6ersity of Copenhagen, 5 Uni6ersitetsparken, DK-2100 Copenhagen, Denmark c Department of Chemistry, Uni6ersity of Arizona, Tucson, AZ 85721, USA d Department of Chemistry, Uni6ersity of Warsaw, Pasteura 1, 02 -093 Warsaw, Poland

Received 16 December 1996

Abstract Vibrational structure of four molecules known to undergo an extremely rapid excited state proton transfer: [2,2%-bipyridyl]-3,3%-diol, 5,5%-dimethyl[2,2%-bipyridyl]-3,3%-diol, [2,2%-bipyridyl]-3-ol and 2-(2-pyridyl)phenol was studied with FTIR and Raman spectroscopy and ab initio quantum chemical calculations. The assignments for all the observed vibrations lying below 600 cm − 1 were proposed, based on the comparison of experimental and computational results of transition energies, shifts upon deuterium and methyl substitution, and the analysis of the evolution of individual bands along the series. The calculations appear to be very reliable in predicting the vibrational frequencies, and in reproducing frequency shifts resulting from deuteration and methylation. The assignment of low-frequency modes may be helpful in understanding of the phototautomerization mechanism, as well as in interpretation of the complicated structure of the band corresponding to the OH stretching vibration. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Vibrational; BP(OH)2; Low frequency

1. Introduction Excited state properties of molecules related to [2,2%-bipyridyl]-3,3%-diol (BP(OH)2) have been the subject of very intense studies over the last decade [1–21]. Large increase of basicity of the pyridine nitrogen and of the acidity of the hydroxyl group * Corresponding author. Fax: + 48 391 20238; e-mail: [email protected]

upon excitation, combined with the presence of double or single intramolecular hydrogen bonds, lead in these compounds to a very rapid phototautomerization involving one or two protons. The spectral manifestation of the process is the appearance of a strongly red-shifted fluorescence, and the lack of emission corresponding to the primarily excited species. The reaction cannot be stopped by deuteration or by lowering of temperature down to the liquid helium region.

1386-1425/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00047-X

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P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

Fig. 1. Formulae and the atom numbering used in the calculations.

The spectral features of BP(OH)2, the most investigated molecule of the series, have led to various applications of hydroxy-heterobiaryls. BP(OH)2 has already found use as a polymer photostabilizer [5], a scintillation counter [20], and a low-temperature quantum-yield standard [7]. The possibility of use in solar energy concentrators has also been considered [6]. For understanding of the excited state proton transfer mechanism, it is important to elucidate the nature of molecular vibrations in these states. It has been suggested for several systems that the low frequency vibrations which change the distance between the hydrogen bonding donor and acceptor atoms may couple to the proton motion and, in fact, determine the tautomerization rate [22]. We have, therefore, undertaken vibrational studies of BP(OH)2 and several related molecules, all of which exhibit excited state intramolecular proton transfer: 5,5%-dimethyl[2,2%-bipyridyl]-3,3%diol (Me2BP(OH)2), [2,2%-bipyridyl]-3-ol (BPOH) and 2-(2-pyridyl)phenol (PP) (Fig. 1). Initial studies [19] focused on the position of the OH stretching band and its correlation with the rates of nonradiative deactivation of the lowest excited singlet states of the phototautomer. In the present work we report the infrared and Raman spectra in the region below 600 cm − 1. The experimental observations are compared with the results of ab initio quantum mechanical calculations of the

ground state vibrational frequencies. Vibrational assignments for all observed low frequency modes are proposed based on (a) good agreement between the experimental and the computed transition energies and intensities; (b) measured and calculated spectral shifts obtained upon deuterium and methyl substitution; (c) analysis of the evolution of individual bands along the series.

2. Experimental The syntheses and purification of the compounds have been described elsewhere [9]. The compounds deuterated at the hydroxyl groups were obtained by shaking the substance with D2O for several hours. The infrared IR absorption spectra in the region above 380 cm − 1 were obtained on the NICOLET SX 170 FTIR spectrometer, with a spectral resolution of 0.5 cm − 1. For room temperature measurements, either KBr tablets or solutions in CS2, CHCl3 or CCl4 were used. Low-temperature measurements were carried out in argon matrices at 13–17 K. The stream of argon containing the heterobiaryl vapours was deposited onto a cold CsI window, attached to a cold finger of a Displex 202 refrigerator (APD Cryogenics). The FIR spectra (250–40 cm − 1) were run on the BRUKER 120 HR FTIR spectrometer.

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

Polyethylene tablets with 3 mg of the sample per 300 mg polyethylene were used. The spectral resolution of this measurement was 2 – 4 cm − 1. FIR spectra (700 – 70 cm − 1) of solutions (CCl4) were run on a Perkin Elmer 2000 FTIR instrument. Polyethylene cells (1 mm) were used and the resolution of this measurement was 1 – 2 cm − 1. The NIR-FT-Raman spectra (3500 – 80 cm − 1) of powdered samples were obtained on the BRUKER IFS Fourier spectrometer (IFS66) equipped wih a FRA 106 Raman module. Spectral resolution was around 6 cm − 1. Most spectra were obtained by accumulating of 500 or 1000 scans. The total registration time was around 30 min with a laser power of 200 – 300 mW at 1064 nm. The Raman spectra were corrected for instrumental response as described previously [23].

1293

RHF/6-31G** level are very similar to those obtained with the 3-21G basis set. The computed bond lengths agree with the X-ray data within 1–2 pm, the bond angles within 1–2°. The largest discrepancy between the results of the calculations and the crystallographic data concerns the dihedral angle between the two rings in PP and BPOH. The former molecule was found planar in the crystal, and the latter was found slightly twisted, by 3.7°. The calculations predict a planar structure for BPOH, while for PP a twisting by 21° is computed. It may well be that the planarity in the crystal is induced by intermolecular forces, similarly to the case of biphenyl [25].

4. Results and discussion

3. Calculations

The IR and Raman spectra are presented in Figs. 2–5 and Tables 1–9. Let us first consider

The quantum mechanical ab initio Restricted Hartree-Fock (RHF) calculations were carried out with the 6-31G** Gaussian basis set implemented in the Gaussian 92 code [24]. The optimal molecular geometries were found varying all the geometrical parameters using the Berny algorithm, also implemented in the Gaussian 92 code. The harmonic frequencies, IR intensites and Raman activities were obtained with the use of the analytical first and second derivatives of the molecular energy with respect to the nuclei displacements from their optimal positions and with respect to the strength of the external electric field. The calculated frequencies were scaled down by the uniform factor of 0.9. The calculated force constant matrices were used in the analysis of the potential energy distributions (PED) in the normal modes. The atom labelling and visualization of the normal modes can be found on the files readable by the HyperChem molecular modeling software. These files can be obtained from the authors upon request. The geometries of BP(OH)2 and BPOH have been previously optimized using STO-3G and 321G basis sets [4]. The structures of the two molecules calculated in the present work at the

Fig. 2. Raman (a) and IR (b) spectra of BP(OH)2. The Raman spectra were measured on polycrystalline samples, the IR spectra are presented for CCl4 solutions.

200, 208 246

457

203

455

566

569

vw

m

vw

m

w

531

582

w

m m

320 351

492

w

m

316

183, 192d

571 585

548

544

519

443 505

439

319 330

296

170 188 248

46 87

a

Atom numbers as in Fig. 1. t, Torsion; g, out-of-plane wag; d, in plane bending; n, stretch; R, both rings. b First column, solutions in CCl4; second column, polyethylene pellet. c Polyethylene pellet. d Crystal splitting.

548, 552d

550

435

m vw

d

m

vw w

83, 89d 103, 125d

86 111

Frequency

Raman frequen- Raman intencyc sity

IR frequencyb IR intensity

Calculated

Experimental

Table 1 Observed and calculated vibrations of BP(OH)2

— —

0.19

11.98

—

0.76 —

14.3

— —

—

— 5.18 0.01

0.95 3.67

IR intensity

10.56 0.22

—

—

1.76

— 3.61

—

7.90 5.34

1.04

5.95 — —

— —

Raman intensity

18ag 6bg

6au

18bu

19ag

7au 7bg

19bu

8bg 20ag

21ag

9bg 20bu 8au

10au 9au

Symmetry

t(R)(96) g(C12C3C2N1)(29), g(C2C13C12N17)(29), t(R1)(17), t(R2)(17) t(R1)(42), t(R2)(42) d(R1)(46), d(R2)(46) t(R1)(31), t(R2)(31), g(O7C2C3C4)(10), g(O18C14C13C12)(10) n(C2C12)(17), d(R1)(11), d(R2)(11), d(O7C3)(14), d(O18C13)(14) t(R1)(36), t(R2)(36) n(C2C12)(17), d(R1)(10), d(R2)(10), d(R)(28) d(O7C3)(43), d(O18C13)(43) t(R1)(44), t(R2)(44) g(C12C3C2N1)(10), g(C2C13C12N17)(10), t(R1)(17), t(R2)(17), g(O7C2C3C4)(20), g(O18C14C13C12)(20) d(O7C3)(23), d(O18C13)(23) d(O7C3)(10), d(O18C13)(10), d(R)(51) g(O7C2C3C4)(30), g(O18C14C13C12)(30) d(R)(66) t(R1)(41), t(R2)(41)

PED contributions (%)a

1294 P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

Raman intensity

91, 99 110

158, 166

278

322

455 486

93

157

279

322

460 485

w m

m

vw

w

vw w

w

w

519

m

404

494

m

342

w

w

274

300

w

w

236

165

510

502

466 469

405

319

308

285

278

260

214

128 140 212

41 43 44 60

Frequency

Raman frequency

IR frequency

IR intensity

Calculated

Experimental

Table 2 Observed and calculated vibrations of Me2BP(OH)2

—

—

3.41 14.2

—

—

5.13

—

0.20

—

—

— 3.14 0.01

0.81 — 1.53 0.48

IR intensity

4.33

4.11

— —

9.35

8.38

—

0.88

—

0.36

2.71

1.10 — —

— 0.51 — —

Raman intensity

23ag

9bg

9au 23bu

10bg

24ag

24bu

25ag

10au

26ag

11bg

12bg 25bu 11au

14au 13bg 13au 12au

Symmetry

t(Me1)(41), t(Me2)(40) t(Me1)(48), t(Me2)(48) t(R)(71) g(R)(36), t(R)(26), t(R1)(11), t(R2)(11) t(R1)(38), t(R2)(38) d(R)(78) t(R1)(28), t(R2)(28), g(C10C4C5C6)(14), g(C21C16C15C14)(14) t(R1)(28), t(R2)(28), g(C10C4C5C6)(10), g(C21C16C15C14)(10) n(C2C12)(17), d(R1)(14), d(R2)(14), d(C10C5)(16), d(C21C15)(16) g(R)(31), t(R1)(22), t(R2)(22) n(C2C12)(10), d(C10C5)(16), d(C21C15)(16) d(C10C5)(27), d(C21C15)(27) d(O7C3)(18), d(O18C13)(18), d(R)(25) t(R1)(19), t(R2)(19), g(C10C4C5C6)(18), g(C21C16C15C14)(18) t(R1)(31), t(R2)(31) d(O7C3)(36), d(O18C13)(36) g(R)(27), t(R1)(15), t(R2)(15), g(O7C2C3C4)(19), g(O18C14C13C12)(19) d(R1)(18), d(R2)(18), d(O7C3)(10), d(O18C10)(18)

PED contributions (%)

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305 1295

See Table 1 for the details.

604

m

561 587

561 587

s m

w

532

531

590

550 590

519

Frequency

Raman intensity

IR intensity

IR frequency

Raman frequency

Calculated

Experimental

Table 2 (continued)

—

14.2 6.84

1.59

IR intensity

4.76

— —

—

Raman intensity

22ag

21bu 8au

22bu

Symmetry

n(C5C10)(10), n(C15C21)(10), d(R1)(31), d(R2)(31) d(R1)(27), d(R2)(27) g(O7C2C3C4)(26), g(C10C4C5C6)(10), g(C21C16C15C14)(10), g(O18C14C13C12)(26) d(R1)(12), d(R2)(12), d(O7C3)(11), d(O18C10)(11)

PED contributions (%)

1296 P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

1297

ences, except for a few cases, did not exceed 20 cm − 1. Actually, the agreement could have been made much better by increasing the frequency scaling factor, 0.90, used for the calculated values. This is because most of the computed values appeared, after scaling, lower than those experimentally observed. However, for consistency, and in order to keep the same factor for mode assignments throughout the whole spectral range, we did not attempt to optimize different scaling factors for different spectral regions. In the second step, we compared the observed and the calculated frequency shifts obtained after replacement of the hydrogen atoms in the hydroxyl groups by deuterium. The results are presented in Tables 3 and 4. As one notices, the agreement between observed and calculated values is impressive: the experimental shifts larger than 5 cm − 1 are reproduced by calculations with the accuracy of 1–3 cm − 1. The only exception is the IR band of Me2BP(OH)2 observed at 455

Fig. 3. Raman (a) and IR (b) spectra of Me2BP(OH)2.

BP(OH)2 and Me2BP(OH)2, two closely related molecules which have the highest symmetry among the members of the series, C2h. The high symmetry is confirmed upon inspection of the spectra (Figs. 2 and 3 and Tables 1 and 2) which show that the IR and Raman selection rules are mutually exclusive. The in-plane modes of BP(OH)2 and Me2BP(OH)2 correspond to ag or bu symmetry species and the out-of-plane modes to au or bg. For BP(OH)2, in the region below 600 cm − 1, the calculations predict 16 vibrations, of which four are of ag symmetry, three of bu, five of au, and four of bg. The corresponding numbers for Me2BP(OH)2 are five (ag), five (bu), seven (au) and five (bg) which yields the total of 22 vibrations. The assignment of the experimentally observed bands to the calculated transitions for BP(OH)2 (Table 1) and for Me2BP(OH)2 (Table 2) proceeded in three steps. First, the observed and calculated energies were compared. It was noticed that the agreement is quite satisfactory: the differ-

Fig. 4. Raman (a) and IR (b) spectra of BPOH.

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

1298

Table 4 Observed and calculated isotope shifts after OH “OD substitution in Me2BP(OH)2

Fig. 5. Raman (a) and IR (b) spectra of PP.

cm − 1, for which the calculations predict a much larger shift than really observed (22 vs 6 cm − 1). This is due to the fact that the vibration calculated for the deuterated species is strongly mixed with another mode of the same symmetry (au), Table 3 Observed and calculated isotope shifts after OH “OD substitution in BP(OH)2 Observed frequency (cm−1)

D6˜ a Observed

83, 89 103, 125 183, 192 208 316 320 351 457 492 531 552 582

0, 0 −1, 0 −1, 1 3 7 1 5 16 0 10 3 5

a

Observed frequency (cm−1)

D6˜ a Observed

D6˜ a Calculated

91 112 165 158 236 274 300 322 342 404 455 486 494 519 532 561 587

−1 −1 1 0 0 2 1 5 10 1 6 12 2 2 1 5 2

0 0 0 1 2 1 3 6 9 4 22 13 6 3 0 6 4

a The difference between the undeuterated and deuterated species.

which describes the out-of-plane OD bending. Moreover, we find that the calculated frequencies of the OH and OD out-of-plane bending modes Table 5 Observed and calculated frequency shifts after double methyl substitution of BP(OH)2, leading to Me2BP(OH)2 Observed frequency (cm−1)

D6˜ a Observed

D6˜ a Calculated

−2 15 27 50 16 84 9 −28 −2 12 −9 −22

2 27 62 48 11 105 11 −30 3 9 −6 −19

D6˜ a Calculated

0 0 1 3 9 3 4 18 −5 11 4 6

The difference between the undeuterated and deuterated species.

BP(OH)2

Me2BP(OH)2

89 125 192 208 316 320 351 458 492 531 552 582

91 110 165 158 300 236 342 486 494 519 561 604

a The difference Me2BP(OH)2.

between

frequencies

in

BP(OH)2

and

w m

325 383 404 503

517

566 578

382

402

502

516

565 574

566 580

518

402 433 504

385

326

a

See captions to Table 1. R, both rings; R2, the ring with the hydroxyl group.

m w

m

m

w

m w

w

w vw w

m

w

w s m

323

190 199 296

190 201 297

181 198 291

m w w

vw m

76 104,120

75 114

559 576

528

413 444 492

363

310

174 191 291

51 91

Frequency

Raman intensity

IR Intensity

IR frequency

Raman frequency

Calculated

Experimental

Table 6 Observed and calculated vibrations of BPOH

4.64 0.44

4.16

1.01 0.19 8.16

0.92

1.53

6.27 0.44 0.68

0.35 4.49

IR intensity

6.35 0.47

1.97

0.57 0.11 0.43

3.51

1.18

0.26 5.05 6.58

0.88 0.30

Raman intensity

34a% 12a%%

13a%%

15a%% 14a%% 35a%

36a%

37a%

38a% 17a% 16a%%

19a%% 18a%%

Symmetry

t(R)(99) g(C2C12C11N16)(36), g(C11C3C2N1)(27), t(R2)(21), t(R1)(10) d(R)(92) t(R2)(52), t(R1)(47) t(R2)(45), t(R1)(23), g(O17C13C12C11)(17), g(C2C12C11N16)(15) n(C2C11)(29), d(R2)(26), d(R1)(24), d(O17C12)(24) d(O17C12)(36), d(R1)(20), d(R2)(20) t(R1)(92) t(R1)(30), t(R2)(55) d(O17C12)(48), d(C2C11C12)(11), d(C3C2C11)(11) g(O17C13C12C11)(41), t(R1)(20), t(R2)(18), g(C11C3C2N1)(15) d(R2)(28) t(R2)(53), t(R1)(14), g(O17C13C12C11)(16), g(C2C12C11N16)(14)

PED contributions (%)a

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305 1299

210 282

326 386 406 427 496 509, 513

555 563, 566

204 277

324

379

401 430

498 508

555 563

556 565

496 510

407 427

384

329

211 288

197

m w

w w

w vw

m

w

m m

w

546 561

491 518

414 436

356

307

207 278

154

a

See captions to Tables 1 and 6. Approximate symmetry, due to nonplanar geometry predicted by calculations.

m s

m w

m m

w

w

w vw

w

191

47 90

189

s

vw w

112

70 114 101

Frequency

Raman intensity

IR intensity

IR frequency

Raman frequency

Calculated

Experimental

Table 7 Observed and calculated vibrations of PP

2.28 8.26

3.42 0.48

1.96 6.24

1.34

0.85

1.37 0.20

0.98

0.09 1.67

IR intensity

6.89 0.84

1.14 1.33

1.07 0.08

1.75

2.58

2.17 5.51

3.37

3.13 1.07

Raman intensity

36a% 13a%%

37a% 14a%%

16a%% 15a%%

38a%

39a%

18a%% 17a%%

40a%

20a%% 19a%%

t(R)(99) g(C2C12C11C16)(38), g(C11C3C2N1)(25), t(R1)(11), t(R2)(17) d(R)(55), t(R1)(14), t(R2)(15) d(R)(28), t(R1)(41) t(R2)(32), t(R1)(26), d(R1)(10), g(C2C12C11C16)(11), g(O17C13C12C11)(11) n(C2C11)(29), d(R2)(26), d(R1)(14) d(O17C12)(18), d(R1)(19), d(R2)(24), t(R2)(12) t(R1)(99) t(R2)(42), t(R1)(15), d(O17C12)(15) d(O17C12)(43) g(O17C13C12C11)(34), g(C11C3C2N1)(14), t(R2)(14), d(R2)(56), n(C11C16)(11) t(R2)(49), t(R1)(18), g(C11C3C2N1)(12), g(C2C12C11C16)(13)

Symmetrya PED contributions (%)

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P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305 Table 8 Observed and calculated isotope shifts after OH “ OD substitution in BPOH Observed frequency (cm−1) 76 104, 120 190 201 297 325 385 404 433 503 517 566 578

D6˜ a Observed

D6˜ a Calculated

0, 2 1 0 1 1 8 3 4 9 1 4 2

0 0 2 0 2 4 7 4 3 9 −3 5 −1

a

The difference between the undeuterated and deuterated species.

are too low, probably due to the fact that the calculations underestimate the influence of the intramolecular hydrogen bonding on these vibrations. Finally, the frequency shifts obtained after double methyl substitution of BP(OH)2 are compared with the theoretically predicted values (Table 5). Table 9 Comparison of observed and calculated frequencies in BPOH and BP(OH)2 Observed frequency (cm−1)

D6˜ a Observed

BPOH

BP(OH)2

75 114 181 199 291 326 385 402 433 502 518 565

86 111 203 183 246 316 351 569 435 455 492 550

a

D6˜ a Calculated

−11 3 −22 16 45 10 34 −167 −2 53 26 15

5 4 −14 21 43 14 33 −135 1 53 23 15

The difference between frequencies in BPOH and BP(OH)2.

1301

In correlating the modes calculated for BP(OH)2 and Me2BP(OH)2, it was helpful to use the onscreen visualization of the calculated forms of vibrations to ensure that the correlation was unequivocal. Again, very good agreement between the theory and the experiment was observed. It should be noted that both positive and negative shifts have been predicted and observed. The assignments obtained based on the OH“ OD and H“CH3 shifts were fully consistent. All in all, assignments of 15 out of 16 calculated vibrations for BP(OH)2 and 19 out of 22 calculated for Me2BP(OH)2 were proposed. These include all in-plane modes (ag and bu species), all but one vibration of bg symmetry, all au modes for BP(OH)2 and all but two for Me2BP(OH)2, respectively. It should be noted that the bands which were not observed correspond to extremely small calculated intensities or should lie in the lowest energy region (Tables 1 and 2). The ground state IR bands of BP(OH)2 and Me2BP(OH)2 lying above 450 cm − 1 and the Raman bands located above 190 cm − 1 have been reported before in the papers devoted mainly to time-resolved resonance Raman spectra [16,17]. The observations and assignments given for BP(OH)2 [16] agree with our present results (a band calculated at 188 cm − 1 which corresponds to bu species has been previously labelled au by mistake [16]). For Me2BP(OH)2, no assignments were proposed thus far. The Raman bands at 494 and 519 cm − 1 (Table 2) were not reported earlier [17], probably due to their low intensity. The largest discrepancies between the observed and calculated frequencies occur for the low frequency region, for which the calculated frequencies were always lower than observed. A possible reason may be due to the fact that these vibrations involve large amplitude motions, mainly torsions, where the assumption of the constant value of the reduced mass may not be strictly valid. The spectra were recorded for polycrystalline samples and solutions. The steric hindrance due to the environment certainly inreases the frequency of such modes with respect to those expected for the isolated molecule. In condensed phases, it is the general rule that the frequencies of out-of-plane and bending vibrations increase in comparison with those of the isolated molecules.

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P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

For some bands, doublets due to crystal splittings were observed (Tables 1, 2, 6 and 7). The correlation diagrams based on crystal symmetry predict splitting into doublets for all molecules in both IR and Raman spectra. The results obtained for BPOH and PP are presented in Figs. 4 and 5 and Tables 6–9. The spectra of the two molecules strongly resemble each other, reflecting a close similarity of the two molecules. As already noted, it is known from X-ray studies [4,13] that PP is planar, and BPOH nearly planar. Interestingly, while the planar crystal structure of BPOH is correctly reproduced, for PP the calculations predict a conformation with the rings twisted by 21° with respect to each other. Such twisting, however, does not seem to greatly influence the computed frequencies, as can be seen from the comparison of Tables 6 and 7. We find the experimental bands corresponding to all of the thirteen vibrations calculated for both molecules to have frequencies lower than 600 cm − 1. In the assignment, one can treat BPOH and PP as lower symmetry analogues of BP(OH)2. In consequence, each band is observed in the IR and in the Raman spectra. The intensities, however, reflect the pattern obeyed for the parent molecule: the bands which are strong in the IR are weak in the Raman spectrum, and vice versa. It should be noted that for all the four compounds the calculations predict well not only the vibrational frequencies, but also the relative intensities. Again, comparison of the observed and computed deuterium shifts was very useful in checking the correctness of the assignments. The data for BPOH are shown in Table 8. Table 9 compares the observed and calculated frequency differences between BPOH and BPOH2. The agreement again is very good. Figs. 5–9 present the forms of the normal modes calculated for BP(OH)2. The calculations reveal that the forms of the vibration remain very similar in all four derivatives. It is, therefore, possible to follow the evolution of vibrational frequencies along the series. Such correlations between the observed and calculated

Fig. 6. The forms of normal modes of ag symmetry calculated for BP(OH)2 (Table 1).

transition energies for the four molecules are shown in Table 10. The agreement between theory and experiment is actually so good that one is tempted to use the calculated shifts to predict positions of the bands which were not identified in the spectra. The only Raman-active bg vibration which was not assigned in BP(OH)2 is expected to be located at 587 cm − 1. Since a strong band is observed at 582 cm − 1, it may possibly overlap with a weak transition, thus making it difficult to observe. Further studies using 15N and 18O species are planned to elucidate this vibration. Armed with the knowledge of the form of normal modes, we can now estimate their possible contribution to the excited state proton transfer reaction coordinate. It is natural to assume that the vibrations important for the process will be those in which the distance between the proton donor and acceptor, i.e. between the oxygen and nitrogen atoms is strongly modified. The actual proton ‘jump’ may occur when these two atoms are close to each other. Only three low

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

Fig. 7. The forms of normal modes of au symmetry calculated for BP(OH)2 (Table 1).

frequency normal modes of BPOH2 seem to be good candidates for the reaction coordinate. These are the 20bu and 19bu species, assigned to the experimental IR bands at 203 and 455 cm − 1,

Fig. 8. The forms of normal modes of bg symmetry calculated for BP(OH)2 (Table 1).

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Fig. 9. The forms of normal modes of bu symmetry calculated for BP(OH)2 (Table 1).

and the 21ag mode assigned to the Raman active vibration observed at 316 cm − 1 (Figs. 6 and 9 and Table 1). It should be noted that the different symmetry of the modes suggests a different mechanism of proton transfer. For the symmetric ag species, the O···N distance is decreased in phase for both pairs of nitrogen and oxygen atoms. The opposite is true for the ungerade vibration: when the O···N separation of one pair decreases, the distance between these atoms on the other pair is increased. It follows that the gerade modes should promote a simultaneous transfer of both protons, while a stepwise mechanism should be favoured if the ungerade vibrations participate in the process. Recent subpicosecond time-resolved fluorescence studies show that both mechanisms are important in the excited state tautomerization of BPOH2 [21]. Both single and double proton transferred tautomers are formed after excitation, on a time scale faster than 300 fs. It should be perhaps recalled here that the frequency of 200 cm − 1 corresponds to the time period of  170 fs.

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

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Table 10 Correlation between vibrations in the four hydroxy-heterobiaryls BP(OH)2

MeBP(OH)2

BPOH

PP

Obsa

Calca

Symm

Obs

Calc

Symm

Obs

Calc

Symm

Obs

Calc

Symm

86 111 246 461 569 203 455 550 316 351 531 582 183 320 492

46 87 248 443 548 188 439 544 296 330 519 571 170 319 505 585

10au 9au 8au 7au 6au 20bu 19bu 18bu 21ag 20ag 19ag 18ag 9bg 8bg 7bg 6bg

93 110

44 60 212 278 466 140 469 550 285 319 510 590 128 214 502 405

13au 12au 11au 10au 9au 25bu 23bu 21bu 25ag 24ag 23ag 22ag 12bg 11bg 9bg 10bg

75 114 291 433 402 181 502 565 326 385

51 91 291 444 413 174 492 559 310 363

19a%% 18a%% 16a%% 14a%% 15a%% 38a% 35a% 34a% 37a% 36a%

70 114 277 430 401 189 498 556 324 379

47 90 278 436 414 154 491 546 307 356

20a%% 19a%% 17a%% 15a%% 16a%% 40a% 37a% 36a% 39a% 38a%

199

191

17a%%

211

207

18a%%

518 574

528 576

13a%% 12a%%

510 563

518 561

14a%% 13a%%

279 460 157 485 561 300 342 519 604 165 236 494 404

Observed, Obs; calculated, Calc; symmetry, Symm. Frequencies (cm−1).

a

5. Summary Combination of spectral and computational techniques turned out to be a powerful method in the analysis of low-frequency vibrations in the series of hydroxy-heterobiaryls. The energies and intensities of vibrational transitions are satisfactorily reproduced by ab initio quantum mechanical calculations. Even more impressive is the agreement between the calculated and observed frequency shifts upon the H“ D and H“ CH3 substitutions, calculated with extremely good accuracy and, therefore, very reliable for the assignment. Good agreement between the experimental spectra and those calculated using ab initio methods similar to the one used in this work has been recently demonstrated for molecules containing six-membered heteroaromatic rings [26]. Our results show that this conclusion can be extended to even larger systems. The knowledge of the low frequency modes is the first step towards the full characterization of the vibrational spectra of heterobiaryls. It will be also used in our future works aimed at the inter-

pretation of the behaviour of the broad and structured band associated with the OH stretching, a vibration which is of crucial importance for the photophysics of BP(OH)2 and related molecules. Finally, the predictions regarding excited state proton transfer reaction coordinate can, hopefully, be checked by using electronic and vibrational time-resolved spectroscopic methods. Such experiments, albeit quite complex, may be quite rewarding in allowing us to follow the elementary steps of ultrafast chemical reactions.

Acknowledgements One of the authors (JW) would like to kindly acknowledge Prof. E.W. Thulstrup and Prof. J. Spanget-Larsen from the Department of Life Sciences and Chemistry, Roskilde University, and their grant from the Danish Natural Science Research Council, for kind permission to use a Perkin Elmer instrument. AL was partly supported from the BST-532/23/96 grant from the Department of Chemistry, University of Warsaw. LA has been supported by the National Science

P. Borowicz et al. / Spectrochimica Acta Part A 54 (1998) 1291–1305

Foundation under grant CHE-9300497. DHC and OFN would like to thank the Danish Natural Science Science Research Council for a general financial support and Haldor Topsoe A/S, Denmark for partly funding the Raman instrument.

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