vibrational analysis of the phenylazonaphthol pigment ca4b

JOURNAL OF RAMAN SPECTROSCOPY, VOL. 29, 421È429 (1998)

Vibrational Analysis of the PhenylazonaphtholPigment 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 commercialphenylazonaphthol pigment Ca4B and the structural analogue (CI 15800 in and and SERRS for SolventH

2O D

2O

Yellow 14 (SY14) in and are presented. The greater signal-to-noise ratio and the advantage of Ñuores-H2O D

2O

cence quenching mean that SERRS gives more information than resonance scattering. The spectra conÐrm that CI15800 is closely related in structure to Ca4B, indicating that the calcium-complexing site in Ca4B is mainlyassociated with the keto and carboxyl groups, with the sulphonic acid group playing a minor part. A semiempiricalcalculation using the PM3 Hamiltonian is used to assign scattering from CI 15800 and Ca4B. The most intenseRaman scattering, due to in-plane modes with the largest displacements on the phenyl and naphthol rings, iscorrectly assigned. Further, the calculation predicts changes due to deuterium exchange of the hydrogen associatedwith the hydrazo group which are borne out by experiment. Hence Raman scattering provides a good in situ probeof 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 dyeshave been extensively investigated but the vibrationalspectra of phenylazonaphthol based dyes less so.1 Thisis despite the fact that they constitute the largest familyof azo dyes used in the textile and food industry andthey are one of the largest families of photoconductivematerials2,3 used in photocopiers and laser printers.

Ca4B toner is the calcium salt of 4-(4-methyl-2-sulphophenyl)azo-3-hydroxy-2-naphthalic acid (alsoknown as Lithol Rubine B, Color Index number15850 : 1, Pigment Red, P.R.57 : 1) and is considered theworldwide standard process red for printing.4,5 Themolecular form of Ca4B can exhibit ketohydrazo toenolazo tautomerism and in solution these tautomersexist in equilibrium with each other. NMR studies showthat the ketohydrazo form dominates in the solidstate.6,7 Additional features believed to be important forthe pigmentary properties of this material are thedimeric structure in the solid state and the hydrogen-bonded network associated with a molecule of water inthe structure. Raman scattering can provide an ideal insitu probe of the molecular structure which gives rise to

* Correspondence to : W. E. Smith, Department of Pure andApplied 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 thehydrogen-bonding network which a†ects packing andother physical properties. This paper is designed toprovide the assignments required for such studies.

The azoÈhydrazo tautomerism of azonaphthol dyeshas been the subject of studies by NMR, UVÈvisible,infrared and Raman spectroscopy.8h11 Several 2-azonaphthol derivatives have been shown to exist inboth the azo and hydrazo forms in the solid state,12although theoretical calculations on 1-phenyl-2-azon-aphthol have predicted that the hydrazo form is ther-modynamically favoured for that molecule.13Electron-withdrawing substituents and hydrogenbonding, both intra- and intermolecular, and the forma-tion of hydrogen-bonded dimers are shown to favourthe hydrazo tautomer.8h18 These Ðndings are consistentwith a Raman spectroscopic study on ionicphenylazonaphthols15 and with a series of papers onthe tautomerization of azonaphthol dyes andpigments9,78h81 which included a Raman spectroscopicstudy,9 where an assignment of the hydrazo modes wasmade. Since then there have been only a few reports onthe resonance Raman spectra of azonaphthol dyes,19h22including one reporting surface-enhanced Raman scat-tering.23 These reports detail the assignments of the azoand the hydrazo Raman bands for the two tautomers. Arecent report has assigned the Raman spectra of twobisazo pigments derived from [email protected] However, these pigments were shown to be inthe azo form, in contrast to most monoazo dyes derivedfrom [email protected]

The assignment of azo bands for azobenzene dyes hasbeen investigated extensively1,25h46 and azobenzene hasbeen 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

422 D. R. ARMSTRONG ET AL .

In contrast, the assignment of hydrazo bands has beenattempted only in a tentative fashion and for hydroxy-azobenzene dyes.22,25,30 A few studies on hydrazosystems have exploited the fact that the hydrazo hydro-gen on the b-nitrogen can be readily exchanged by deu-terium by solvating the dye in a deuterated solvent, e.g.

The subsequent changes observed in theD2O.19,22Raman spectra help to assign the bands that involve thehydrazo group.

This paper reports Raman scattering following deute-rium exchange for the commercial pigment Ca4B. Inaddition, two related systems were studied, the dyes CI15800 and Solvent Yellow 14 (SY14) (CI 12055) (Fig. 1).Ca4B has three groups which may bond with thecalcium and/or contribute to the hydrogen-bondingnetwork and so a†ect the solid-state structure. The sul-phonic acid group is not present in CI 15800 andneither the sulphonic acid group nor the carboxylategroup is present in SY14. Comparison of the threesystems enables the relevance to structure of each groupto be assessed.

Resonance Raman scattering was recorded fromCa4B and from CI 15800 but owing to Ñuorescence aspectrum from SY14 could not be recorded. However,surface-enhanced resonance Raman scattering (SERRS)from colloidal silver in both water and wasD2Orecorded for all three compounds. SY14 was the subjectof a recent SERRS investigation and this study makessome use of the data obtained.48 Fluorescence quen-ching in SERRS has been known for some time but wasfound to be widespread in azo dyes recently.49

To aid in the deÐnition of the assigned modes, molec-ular orbital calculations were carried out for all threemolecules. Semiempirical methods [modiÐed neglect ofdiatomic overlap (MNDO), Austin model 1 (AM1) andparametric method 3 (PM3)] were found, following suit-

Figure 1. Structures of SY14, CI 15800 and Ca4B.

able correction, to yield results comparable to thosefrom ab initio methods for the hydrazo structure inSY14. These semiempirical methods require a fractionof the computing time of ab initio methods50h52 and aremore suitable for calculations on large molecules suchas dyes and were employed in this study. It should benoted that these calculations are only suitable for thehydrazo tautomer of phenylazonaphthols. The PM3calculations do not predict azo vibrations accurately,47although a more recent calculation indicates that thedensity matrix method is more e†ective.53

EXPERIMENTAL

SY14, sodium citrate (Aldrich) and silver nitrate(Johnson Matthey) were of analytical grade. Ca4B wassynthesized by the diazotization of 2-amino-5-methyl-sulphonic acid (4B acid), coupling to 3-hydroxy-2-naph-thoic acid (BONA) and slaking with calcium chloride,using standard pigment procedures.4,5 The synthesis ofCI 15800 was very similar to that of Ca4B, except thataniline was used in place of 4B acid. CI 15800 was notslaked and the free acid was used in the experiments.CHN analyses were within 0.4% of the theoreticalvalue.

Citrate-reduced colloidal silver was prepared accord-ing to the Lee and Meisel method.52 A 90 mg amountof silver nitrate in 500 ml of distilled water was rapidlyheated to 100 ¡C, 10 ml of a 1% solution of sodiumcitrate were added with vigorous stirring and the solu-tion was kept at 100 ¡C for 1 h under constant stirring.

Ca4B and CI 15800 are both slightly soluble in waterand dilute suspensions, equivalent if fully dissolved to asolution of a concentration of 10~4 M, were added tothe colloidal silver to obtain SERRS. For the deuteriumexchange experiments, Ca4B and CI 15800 were dis-persed in Colloidal silver was centrifuged at 4000D2O.rpm for 1 h and the supernatant replaced with ToD2O.this were added Ca4B and CI 15800 dispersed in D2O.The Ðnal concentration of the dyes was approximately10~4 M.

SY14 is less soluble in water and is soluble in ethanol.To observe SERRS, SY14 was dissolved in a 50 : 50mixture of water and ethanol to 10~4 M and a drop wasadded to 2 cm3 of colloidal silver. For the deuteriumexchange experiment the colloidal silver was centrifugedand the supernatant replaced with To this wasD2O.added a drop of a SY14 dissolved in a 50 : 50 mixture of

and ethanol at a concentration of 10~4 M. TheD2OÐnal concentration of all compounds in the colloidalsilver was between 10~5 and 10~6 M, which, givenefficient surface adsorption, would result in abovemonolayer coverage of the compounds on the silversurface.

Raman spectra were obtained on a modiÐed Cary 81double monochrometer equipped with a thermoelectri-cally cooled photomultiplier tube and photon countingdetection system. The monochromatic excitation sourcewas a Spectra-Physics Model 2020 argon ioncontinuous-wave laser using 457.9 nm radiation at 100mW. The slit width was 5 cm~1.

The semiempirical calculations for hydrazone tauto-meric forms of SY14 and CI 15800 were performed with

( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

VIBRATIONAL ANALYSIS OF PHENYLAZONAPHTHOL PIGMENT Ca4B 423

the MOPAC network of programs (version 6) using thePM3 Hamiltonian. The calculated wavenumbers weremultiplied by 0.89 to correct for anharmonicity.

RESULTS AND DISCUSSION

Resonance Raman scattering of Ca4B and CI 15800 inandH

2O D

2O

Resonance Raman scattering from suspensions of Ca4Bdispersed in and (Fig. 2) is similar to thoseH2O D2Oobtained from other ketohydrazo dyes. At 10~4 M thereis some solubility apparent in the suspensions. This“bleedÏ is small and the spectrum is identical with poorersignal-to-noise ratio spectra from KBr discs so that it isthought to be due predominantly to the solid state. Themost noticeable di†erences between the spectra in Fig. 2are that the peak at 1228 cm~1 in is replaced withH2Oone at 1212 cm~1 in (Table 1) and the peak at 962D2Ocm~1 in is replaced with one at 934 cm~1 inH2O D2O.The spectra and the di†erences between andH2O D2Osuspensions indicate an exchangeable hydrogen on anNÈH bond, conÐrming that Ca4B in the solid state isin the hydrazone form. No appreciable di†erence isobserved in the resonance Raman spectra of these com-pounds at pH/D 13 in either or where theH2O D2O,azo form dominates (data not shown).

CI 15800 has a similar solubility to Ca4B in andH2Oand Raman scattering was obtained under theD2Osame conditions (Fig. 3). Wavenumber shifts upon deu-terium exchange are similar to those for Ca4B (Table 1).In particular, both CI 15800 and Ca4B have peaks atabout 1230 and 960 cm~1 in which are replacedH2Oby peaks at about 1215 and 935 cm~1 in ThereD2O.are intensity di†erences but the similarities suggest that

Figure 2. Resonance Raman spectra of 10É3 M Ca4B at pH 7.Excitation at 457.9 nm. (A) Dispersed in (B) dispersed inH

2O;

D2O.

Figure 3. Resonance Raman spectra of 10É2 M Cl 15800 at pH.Excitation at 457.9 nm. (A) Dispersed in (B) dispersed inH

2O;

D2O.

the NÈH bond and associated hydrogen-bondingnetwork are very similar in the two compounds.

Surface-enhanced resonance Raman scattering of Ca4B,CI 15800 and SY14

Resonance Raman scattering from SY14 could not beobtained from aqueous solutions on suspension because

Table 1. Resonance Raman peak positions for CI 15800 andCa4B in andH

2O D

2Oa

Ca4B CI 15 800

H2O D

2O H

2O D

2O

l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1

1604 st 1602 st 1600 st 1600 st

1560 st 1555 st 1560 m 1558 m

1490 st 1489 st 1495 st 1495 st

1455 m 1454 m 1455 w 1458 m

1390 st 1390 m 1388 m 1390 m

1362 st 1358 st 1365 st 1365 st

1328 w 1328 w 1326 w 1326 w

1260 m 1260 m 1265 m 1264 m

1228 m 1212 m 1232 m 1215 m

1180 m 1179 m 1180 m 1180 st

1160 w 1160 w 1160 w 1160 w

1115 m 1120 m

1020 w 1035 w 1045 w 1040 w

1000 w 1018 w 1012 w

962 m 934 w 968 m 940 w

888 w 875 w

820 w 850 w

785 w 800 w 800 w

740 w 740 w 750 w 750 w

708 w 708 w 718 w 718 w

660 w

600 w 600 w

500 w 500 w 475 w 470 w

a st, Strong; m, medium; w, weak.



it is virtually insoluble in water and does not suspendwell, presumably because the surfaces of the particlesare non-polar and do not wet. Further, the solid-statespectra obtained from KBr or silver discs are of verypoor quality owing to intense Ñuorescence. The Ñuores-cence quenching associated with SERRS makes it pos-sible to obtain good spectra from SY14 in addition toCa4B and CI 15800. Colloidal SERRS requires adher-ence of the particles or molecules to the silver colloidsurface which is negatively charged. Thus, wetted (polar)pigment particle surfaces or some solubility is required.The similarities between the SERRS and resonance datasuggest that particles rather than molecules dominatethe spectrum. They must either adhere to the surface orbe reformed from adsorbed molecules from the bleed onthe surface (Figs 4 and 5). To obtain solubility for SY14,measurements were carried out in silver colloid sus-pended in 50 : 50 waterÈethanol as solvent. To obtaindeuterated SERRS, the colloidal silver was centrifuged,the water removed and the colloid resuspended in D2O.This is a technique that has proved useful in obtainingSERS from deuterium-exchanged molecules and mol-ecules which otherwise would not give SERS owing toinsolubility in water.54,55

SERRS (Figs 4È6 and Table 2) gave a very highsignal-to-noise ratio and revealed more informationthan resonance Raman scattering. These are di†erencesparticularly in the low-energy region compared withCa4B and CI 15800. Further, SERRS for SY14 (Fig. 6)indicates di†erences in intensity and wavenumber ondeuterium exchange compared with Ca4B and CI15800. All compounds show a deuterium-sensitive

Figure 4. SERRS of 10É4 M Ca4B from colloidal silver. Excitationat 457.9 nm. (A) Dispersed in (B) dispersed inH

2O; D

2O.

Figure 5. SERRS of 10É4 M CI 15800 from colloidal silver. Exci-tation at 457.9 nm. (A) Dispersed in (B) dispersed inH

2O; D

2O.

Figure 6. SERRS of 10É4 M SY14 from colloidal silver. (A) Dis-persed in excitation at 514.5 nm; (B) dispersed in exci-H

2O, D

2O,

tation at 457.9 nm.



Table 2. SERRS wavenumbers for Ca4B, CI 15800 and SY14 inandH

2O D

2O

Ca4B CI 15800 SY14

H2O D

2O H

2O D

2O H

2O D

2O

l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1

1605 st 1608 st 1598 st 1600 st 1595 st 1652 w

1552 st 1555 st 1554 m 1558 st 1555 m 1610 m

1490 st 1490 st 1495 st 1492 st 1495 st 1600 st

1449 m 1455 m 1450 m 1455 m 1480 st 1552 m

1390 m 1412 w 1395 m 1395 st 1450 nm 1495 st

1362 st 1359 st 1365 st 1366 st 1484 st

1332 st 1332 m 1330 m 1330 m 1455 m

1290 m 1275 m 1425 m

1260 st 1260 m 1260 m 1260 m 1385 st 1380 st

1234 st 1236 st 1345 st 1342 m

1219 m 1211 st 1315 w 1311 w

1190 m 1265 m 1265 m

1176 st 1180 st 1178 st 1181 st 1235 st 1212 st

1160 m 1160 m 1155 w 1160 m 1215 m

1138 w 1180 m 1170 m

1114 w 1118 m 1112 m 1120 m 1150 w 1160 w

1092 w 1092 w 1146 w

1042 w 1038 w 1042 w 1040 m 1100 m 1105 m

1020 w 1008 w 1020 w 1016 m 1080 w 1080 w

960 st 960 st 1060 w

936 w 939 m 1040 w 1038 w

890 w 880 w 870 w 882 w 1005 w 1000 w

850 w 862 m 990 m 965 w

820 w 820 w 925 w

780 w 782 w 775 w 872 w 875 w 870 w

745 w 742 m 748 m 748 w 845 w 840 w

710 w 715 w 712 w 718 w 790 w

682 w 660 w 768 w

664 w 664 w 662 w 650 w 735 m 730 m

600 w 602 w 602 w 602 w 620 w 628 w

570 w 570 w 595 w 585 w

554 w 553 w 550 w 552 w 550 w 542 w

532 w 530 w 510 w 500 w

515 w 510 w 500 w 498 w 475 m 464 m

495 w 495 w 430 w 438 w

470 w 465 w 468 w 465 w 420 w 415 w

445 w 445 w 440 w 440 w 370 w 370 w

426 w 425 w 352 w

370 w 365 w 385 w 385 w 312 w

330 w 340 w 368 w 364 w 352 w

310 w 305 w 314 w 304 w 227 w 300 w

265 w 260 w 250 w 250 w 238 w

220 w 220 w 224 m

SERRS peak at about 1235 cm~1 in but SY14H2Oshows di†erences in the other deuterium exchange sen-sitive peaks when compared with Ca4B and CI 15800.The conclusion reached is that the solid-state structureof SY14 di†ers more profoundly from the other two andthat the carboxyl group plays a larger part than thesulphonic acid group in controlling the structure ofCa4B. The proximity of the carboxyl and keto groupsand the affinity of calcium for them make this reason-able.

Normal-mode analysis of CI 15800 and SY14

Normal-mode displacements and wavenumbers for CI15800 obtained from semiempirical calculations withthe PM3 Hamiltonian were used to assign the spectra of

Figure 7. Numbering scheme used for CI 15800 in PM3 semi-empirical calculations and in describing normal modes.



CI 15800 and Ca4B. The numbering of atoms is givenin Fig. 7. The calculation predicts a non-planar molecu-lar geometry. The bond has a considerableC8xO17positive z component, where z is the axis perpendicularto the plane of the paper (the xy plane). The benzenepart of the molecule is not planar with the naphthalenemoiety, but slightly twisted with respect to it. However,for simplicity the molecule has been drawn as planar forthe normal-mode diagrams.

The predicted wavenumbers scaled to compensate foranharmonicity are given in the range 1700È700 cm~1for CI 15800 and SY14 and the correspondingdeuterium-exchanged compounds (Table 3). Asobserved in the Raman spectra, many of the modes ofboth dyes do not change upon deuterium exchangewhereas a few are diagnostic of the change. The modesplaced together in Table 3 exhibit the same energy con-tributions from the speciÐc atoms in both the non-deuterated and deuterated molecules. Those placed withno mode opposite exhibit unique patterns of energycontributions. For example, even though the mode pre-dicted at 1224 cm~1 for SY14 is very close in wavenum-ber to the 1225 cm~1 mode observed upon deuteriumexchange, the energy contributions are not the same.This mode involves the hydrazo part of the moleculeincluding the exchangable H in the H(20)ÈN(19) bondwhich makes a dominant contribution and the form ofthe mode changes upon deuterium exchange. Overall,the changes predicted are consistent with thoseobserved experimentally and give conÐdence in the useof the calculation in assigning the spectra.

Assignment of CI 15800 and Ca4B vibrational spectra

The two highest energy non CÈH stretching modes, l82and can be assigned to the CO stretch and thel81,asymmetric stretch, respectively. The CO stretchCO2~cannot be assigned to any of the observed bands of CI15800 or Ca4B as no Raman bands are observed above1605 cm~1. However, Ca4B does have an IR band at1655 cm~1 and is assigned to the asymmetricl81 CO2vibration. is centred on the naphthol part of thel80molecule, with a large C(9)ÈC(10) stretch contributionand has been assigned to the 1620 cm~1 IR active bandof Ca4B and the 1618 cm~1 IR active band of CI 15800.

The CÈC(21È26) phenyl ring can be considered as amonosubstituted phenyl unit which displays phenylmode character in the modes of CI 15800. The modes

all show similarities to the benzene model79Èl758a/b.55 Additionally, modes and have a largel79 l78C(8)ÈN(18) (CxN) stretch contribution with havingl79a larger contribution than Both of these modes arel78 .una†ected by deuterium substitution of the b-nitrogenof the hydrazo. Previous studies on 1-phenyl-2-azon-aphthol dyes have incorrectly assigned the CxN stretchto peaks sensitive to deuterium substitution19,22 Thesetwo modes exhibit a phase pair relationship for the CÈC(1È6) and CÈC(21È26) rings which relates to benzenemode 8a/b. In mode the CÈC(1È6) unit is vibratingl78in the opposite phase to mode while other relatedl79 ,vibrations are in a similar sense for both modes. Thisphase pair relationship is a repeated feature of severalother modes of CI 15800. Modes are dominatedl74Èl71by CÈC(1È6) or CÈC(21È26) displacements and can be

Table 3. PM3 predicted wavenumbers for CI15800 and SY14

CI 15800 SY 14

NH ND NH ND

l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1

l82

1747 l82

1747 l74

1705 l74

1703

l81

1699 l81

1699 l73

1661 l73

1617

l80

1648 l80

1648 l72

1607 l72

1607

l79

1608 l79

1608 l71

1600 l71

1599

l78

1596 l78

1596 l70

1592 l70

1592

l77

1589 l77

1589 l69

1505 l69

1501

l76

1578 l76

1578 l68

1467 l68

1466

l75

1578 l75

1576 l67

1430 l67

1430

l74

1448 l74

1448 l66

1416 l66

1420

l73

1406 l73

1405 l65

1394 l65

1403

l72

1403 l72

1402 l64

1347

l71

1381 l71

1378 l63

1306 l64

1306

l70

1340 l70

1340 l62

1289 l63

1290

l69

1264 l62

1225

l69

1268 l61

1224

l68

1251 l60

1211 l61

1210

l67

1233 l68

1234 l59

1191 l60

1193

l67

1216 l58

1176 l59

1183

l66

1200 l57

1152 l58

1148

l65

1181 l66

1181 l56

1113 l57

1114

l65

1175 l55

1110 l56

1109

l64

1170 l64

1169 l54

1095 l55

1096

l63

1159 l53

1056 l54

1056

l62

1098 l63

1100 l52

1046 l53

1046

l61

1082 l62

1081 l51

1040 l52

1040

l60

1069 l61

1069 l51

1035

l59

1044 l60

1044 l50

1027 l50

1026

l59

1034 l49

1015

l58

1030 l58

1030 l49

1011 l48

1011

l57

1027 l57

1028 l48

1002 1002

l56

1016

l55

1000 l56

1000 l47

1000 l47

1000

l54

998 l55

998 l46

978

l53

987 l54

987 l46

960

l52

985 l53

985 l45

939

l52

957 l45

939

l51

945 l44

927

l50

920 l51

915 l44

915 l43

915

l49

894 l50

985 l43

912 l42

912

l48

983 l49

893 l42

902 l41

902

l48

887 l41

878 l40

878

l47

865 l40

868 l39

868

l46

864 l47

864

l45

855 l46

859

l45

852 l39

838 l38

837

l44

838 l44

838 l38

817

l43

823 l43

822 l37

811 l37

811

l42

800 l42

799 l36

797 l36

800

l41

788 l35

793

l40

785 l41

786 l35

778 l34

777

l40

761 l34

759 l33

759

l39

748 l39

748

l38

739 l38

738 l33

741 l32

740

l37

716 l37

716 l32

709 l31

709

assigned as related to benzene mode 19a and 19b. Thefull assignment of 1600È1300 cm~1 modes is given inTable 4.

Modes do not show any recognizablel69Èl66benzene mode character (Fig. 8). However, they doshow varying degrees of NÈH stretching and bendingcontributions. Indeed, modes and arel69 , l68 l66



Table 4. Band positions and assignments for Ca4B and CI 15800

Ca4B CI 15800

IR SERRS IR SERRS Mode Assignment

l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l l8 /cmÉ1 Descriptiona

1655 m l81

1699 CO2

asymmetric

1620 m 1618 st l80

1648 C–C(1–10)

1601 m 1605 st 1595 st 1598 st l79

1608 N(18)–C(7), C–C(1–6) 8a/b1561 st 1569 st l

781596 N(18)–C(7), C–C(1–6), 8a/b

1550 st 1552 st 1553 st 1554 m l77

1589 C–C(1–6), 8a/b1498 st 1490 st 1496 st 1495 st l

761578 C–C(1–6), 8a/b

1482 st 1482 st l75

1578 C–C(20–26), 8a/b1450 st 1449 m 1448 st 1450 m l

741448 C–C(1–6), 19b

1407 st 1398 st l73

1406 C–C(20–26), 19b

1389 m 1390 m 1389 st 1395 m l72

1403 C–C(20–26), 19b

1365 m 1362 st 1356 m 1365 st l71

1381 C–C(20–26), 19b

1324 m 1332 st 1326 m 1330 m l70

1340 CO2

symmetric

1288 st 1290 m 1275 m l69

1268 C–C, N–H

1265 st 1260 st 1267 st 1260 m l67

1233 C–C(1–10)

1249 st 1234 st 1233 w 1236 st l68

1251 C–C, N–H

1211 st 1190 m l66

1200 C–C(21–26), N–H

1185 st 1176 st 1187 st 1178 st l65

1181 C–C(1–6), 14

1158 m 1160 m 1146 st 1155 w l64

1170 C–C(21–26), 14

1138 w l63

1159 C–C(21–26), 14, N–H

1128 w 1114 w 1112 m l62

1098 C–C(21–26), 3

1091 m 1092 w 1091 w l61

1082 C–C(1–6), 3

1075 w 1076 w l60

1069 C–C(1–10), N–N

1030 m 1042 w 1039 m 1042 w l59

1044 C–C(21–26), 9a

1020 st 1020 w 1013 st 1020 w l56

1016 C–C, N–N

954 m 960 st 956 m 960 st l51

945 C–C(21–26), 18a, N–H

877 m 890 w 870 w l48

893 C–C(1–6), 5

864 m 850 w l45

856 C–C(1–6), 11

821 st 820 w 825 st l43

823 C–C(21–26), 17b

786 m 780 w 785 w 775 w l41

788 C–C(21–26), 12, N–H

766 st 765 st l40

785 C–C(21–26), 12

749 st 745 w 748 st 748 m l39

748 C–C(21–26), 10a

699 m 710 w 712 w l37

716 C–C(1–10)

685 m 682 w l35

686 C–C(1–16), 11

664 w 662 w l34

673 C–C(1–10)

645 w l35

583 C–C(1–10)

626 m 618 w 615 w l32

615 C–C(1–10), N–H

612 m 600 w 595 m 602 w l30

583 C–C(1–6), 6a

570 w l29

565 C–C(21–26), 16a

554 w 550 w l28

559 C–C(21–26), 6b

520 m 530 st 532 w l27

543 C–C(1–10)

506 m 515 w 492 st 500 w l26

516 C–C, N–N

494 m 495 w l25

511 C–C(1–10)

470 w 468 w l24

478 C–C(1–10), N–N

445 w 440 w l22

437 C–C(1–10)

416 st 426 w 416 st l21

422 C–C(21–26), 4

370 w 385 w l19

394 C–C(21–26), N–N

330 w 368 w l18

333 C–C(1–10)

310 w 314 w l16

300 C–C(1–6)

265 w 250 w l14

261 C–C, N–N

220 w l12

216 C–C(1–10)

a Atoms contributing most to the mode are given in the last column, followed by the parentbenzene mode55 where appropriate.

heavily dependent on the NH group as these vibrationsare signiÐcantly altered upon deuterium exchange(Table 3). These modes also show a large contributionfrom the C(21)ÈN(19) stretch. Therefore, and arel69 l68assigned to the deuterium-sensitive Raman bands ofCa4B and 1290 and 1234 cm~1, respectively. Although

has a contribution from an NÈH wag, this mode isl67not predicted to change upon deuterium substitutionand is therefore assigned to the Ca4B Raman band at1260 cm~1. The ability of the PM3 calculation to

predict the observed changes in the spectra upon deute-rium substitution is surprisingly good and gives furtherconÐdence in the assignment.

The di†erences in the Raman spectra of thedeuterium-sensitive bands between Ca4B and CI 15800are due to the di†erences caused by the sulphonate andmethyl groups on the phenyl ring of Ca4B. The sul-phonate in particular may a†ect these vibrations as thisgroup will be involved in the hydrogen-bondingnetwork involving the hydrazo NH. This may explain



Figure 8. PM3 calculated CI 15800 normal-mode displacements that display hydrazo character.

the di†erence in the wavenumber of the Raman bandsassigned to which has a large contribution from thel69 ,NÈH wag.

has a large contribution from NÈH wag and alsol63from phenyl mode 14, and is also noticed to transformupon deuterium substitution. Therefore, this mode isassigned to the weak Raman band at 1138 cm~1 ofCa4B. There is one other noticeable Raman bandpresent at 960 cm~1 in both Ca4B and CI 15800. Thisband is assigned as which has a contribution froml51,phenyl mode 18a.

The low-wavenumber vibrations are difficult to assignowing to the large number and close spacing of thewavenumbers. However, a tentative assignment hasbeen attempted for CI 15800 and Ca4B (Table 4). Manyof these vibrations are complex out-of-plane modes withlittle or no recognizable phenyl mode character.

CONCLUSIONS

Both the experimental results and theoretical calcu-lations show that CI 15800 is a better model than SY14for Ca4B. The similarity of the CI 15800 and Ca4Bspectra and the di†erence between them and the spec-trum of SY14 indicate the importance of the carbox-ylate group in determining structure.

The normal-mode calculations reveal that the Ramanactivity of the high-wavenumber modes comes frommodes with large contributions from the phenyl ringand the second naphthalene ring [CÈC(1È6) and CÈC(21È26), respectively]. The hydrazo vibrations are alsoclear and deÐned in the normal-mode calculations andRaman spectra by exchanging the NH hydrogen for



deuterium. The wavenumber of the CxN stretch is notsensitive to deuterium exchange and is predicted at 1608cm~1. The deuterium-sensitive modes of the hydrazogroup involve deformation of the CÈNÈH bond, as in

and In mode an NÈN stretch isl69 , l68 l66 . l56involved in addition to an NÈH deformation. There is avery good Ðt between theory and experiment, validatingthe use of the PM3 calculation in assigning the spectraand conÐrming the form of the vibrations observedexperimentally. The hydrogen-bonding network is a keyfeature of the solid state structure and properties ofCa4B. For example, the commercial system has onemolecule of water per dimer and removal of this wateralters the structure. Raman scattering can now be usedto probe this change informatively.

In both the resonance and SERRS spectra of Ca4Band CI 15800, it is the in-plane high-energy modes thatare the most intense. These modes involve either the

phenyl or the naphthol part of the molecule or bothring systems. The dominance of the phenyl part in someof the resonance Raman active modes is unexpected. Itwas thought that only the naphthol-dominated modeswould contribute to the resonance scattering since onlythe naphthol and hydrazo parts were planar and clearlyconjugated and thus might form the e†ective chromo-phore. The results indicate that the chromophoreextends over the whole molecule.

The out-of-plane low-energy modes are weak in reso-nance but are easily detectable by SERRS. SERRS wasclearly related to resonance scattering but enabledmuch more vibrational information to be obtainedowing to the higher signal-to-noise ratio. In addition,the Ñuorescence quenching inherent in the processextended the range of molecules which could be studiedwith excitation at resonant wavenumbers. It could beused more widely in studies of dyes and pigments.

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vibrational analysis of the phenylazonaphthol pigment ca4b

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