JOURNAL OF RAMAN SPECTROSCOPY, VOL. 29, 421È429 (1998) Vibrational Analysis of the Phenylazonaphthol Pigment Ca4B J. Clarkson,¤ D. R. Armstrong, C. H. Munro” and W. E. Smith1* Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK Resonance Raman scattering and surface-enhanced resonance Raman scattering (SERRS), for the commercial phenylazonaphthol pigment Ca4B and the structural analogue (CI 15800 in and and SERRS for Solvent H 2 O D 2 O Yellow 14 (SY14) in and are presented. The greater signal-to-noise ratio and the advantage of Ñuores- H 2 O D 2 O cence quenching mean that SERRS gives more information than resonance scattering. The spectra conÐrm that CI 15800 is closely related in structure to Ca4B, indicating that the calcium-complexing site in Ca4B is mainly associated with the keto and carboxyl groups, with the sulphonic acid group playing a minor part. A semiempirical calculation using the PM3 Hamiltonian is used to assign scattering from CI 15800 and Ca4B. The most intense Raman scattering, due to in-plane modes with the largest displacements on the phenyl and naphthol rings, is correctly assigned. Further, the calculation predicts changes due to deuterium exchange of the hydrogen associated with the hydrazo group which are borne out by experiment. Hence Raman scattering provides a good in situ probe of the hydrogen-bonded network essential to the properties of these compounds. 1998 John Wiley & Sons, Ltd. ( J. Raman Spectrosc. 29, 421È429 (1998) INTRODUCTION The Raman spectra of azobenzene and related dyes have been extensively investigated but the vibrational spectra of phenylazonaphthol based dyes less so.1 This is despite the fact that they constitute the largest family of azo dyes used in the textile and food industry and they are one of the largest families of photoconductive materials2,3 used in photocopiers and laser printers. Ca4B toner is the calcium salt of 4-(4-methyl-2- sulphophenyl)azo-3-hydroxy-2-naphthalic acid (also known as Lithol Rubine B, Color Index number 15850 : 1, Pigment Red, P.R.57 : 1) and is considered the worldwide standard process red for printing.4,5 The molecular form of Ca4B can exhibit ketohydrazo to enolazo tautomerism and in solution these tautomers exist in equilibrium with each other. NMR studies show that the ketohydrazo form dominates in the solid state.6,7 Additional features believed to be important for the pigmentary properties of this material are the dimeric structure in the solid state and the hydrogen- bonded network associated with a molecule of water in the structure. Raman scattering can provide an ideal in situ probe of the molecular structure which gives rise to * Correspondence to: W. E. Smith, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK. E-mail : w.e.smith=strath.ac.uk ¤ Present address : Department of Chemistry, Leeds University, Leeds, UK. ” Present address : Department of Chemistry, University of Pitts- burgh, Pittsburgh, PA, USA. the visible adsorption and of the nature of the hydrogen-bonding network which a†ects packing and other physical properties. This paper is designed to provide the assignments required for such studies. The azoÈhydrazo tautomerism of azonaphthol dyes has been the subject of studies by NMR, UVÈvisible, infrared and Raman spectroscopy.8h11 Several 2- azonaphthol derivatives have been shown to exist in both the azo and hydrazo forms in the solid state,12 although theoretical calculations on 1-phenyl-2-azon- aphthol have predicted that the hydrazo form is ther- modynamically favoured for that molecule.13 Electron-withdrawing substituents and hydrogen bonding, both intra- and intermolecular, and the forma- tion of hydrogen-bonded dimers are shown to favour the hydrazo tautomer.8h18 These Ðndings are consistent with a Raman spectroscopic study on ionic phenylazonaphthols15 and with a series of papers on the tautomerization of azonaphthol dyes and pigments9,78h81 which included a Raman spectroscopic study,9 where an assignment of the hydrazo modes was made. Since then there have been only a few reports on the resonance Raman spectra of azonaphthol dyes,19 h22 including one reporting surface-enhanced Raman scat- tering.23 These reports detail the assignments of the azo and the hydrazo Raman bands for the two tautomers. A recent report has assigned the Raman spectra of two bisazo pigments derived from benzene-2@-azonaph- thols.24 However, these pigments were shown to be in the azo form, in contrast to most monoazo dyes derived from benzene-2@-azonaphthols.4 The assignment of azo bands for azobenzene dyes has been investigated extensively1,25 h 46 and azobenzene has been the subject of a recent study in this laboratory.47 CCC 0377È0486/98/050421È09 $17.50 Received 21 October 1996 ( 1998 John Wiley & Sons, Ltd. Accepted 8 February 1998