Surface-Enhanced Raman Spectroscopic and Density Functional

J. Phys. Chem. B 2002, 106, 101-112

101

Surface-Enhanced Raman Spectroscopic and Density Functional Theory Study of Adenine Adsorption to Silver Surfaces Bernd Giese and Don McNaughton* School of Chemistry, Monash UniVersity, Wellington Rd, Clayton, Victoria, Australia ReceiVed: February 28, 2001; In Final Form: September 5, 2001

The surface-enhanced Raman spectra (SERS) of adenine and three deuterated analogues adsorbed on colloids, electrochemically roughened electrodes, and vacuum deposited island films of silver have been investigated. All normal Raman and SERS bands were assigned to normal modes on the basis of density functional theory (DFT) calculations (B3LYP/6-31++G(d,p)) and isotope shifts. Surface selection rules derived from the electromagnetic enhancement model were employed to deduce adenine orientations on the different surfaces. On the colloids, adenine adopts an almost perpendicular orientation interacting with the metal surface via N7 and the exocyclic amino group. On the electrodes, adenine adsorbs in a more tilted orientation while on the island films the tilt is even more pronounced. Interaction with the electrodes takes place through N7 and the amino group, while interaction with the island film may be solely through N7.

1. Introduction Conventional Raman spectroscopy of biomolecules is limited by the inherent weak intensity of Raman scattered light and the interference of fluorescence. One way to overcome these problems is surface-enhanced Raman spectroscopy (SERS).1-4 If an analyte is adsorbed to a rough metal surface, the Raman cross section may be enhanced by a factor of up to 1014 (refs 5-7) and fluorescence may be quenched.1 Despite its widespread application, the SERS effect is still not completely understood, although there is general agreement that at least two enhancement mechanisms are at work.2 In the electromagnetic (EM) models1,8,9 the enhancement is caused by resonance between the incident or Raman scattered light with surface plasmons of the metal substrate. A simple EM model proposed by Creighton8,9 is based on a spherical metal particle that is substantially smaller than the wavelength of the incident light. The incident electric field induces a displacement in the conduction electrons of the sphere, and an oscillating dipole is generated that in turn radiates a secondary field. The resulting electric field on the particle surface is the sum of the incident and secondary fields. Since the conduction electrons are spatially confined they have a characteristic frequency and a quantized energy called a plasmon. Coincidence of the energy of either the incident light or the Raman scattered light with the plasmon increases the secondary field considerably, which in turn enhances the Raman signal. More recent EM models treat the surface substrates as fractal surfaces rather than spheres.10,11 On the basis of the electromagnetic enhancement model, socalled surface selection rules have been derived.9 Those modes that involve a large change in polarizability perpendicular to the surface are enhanced the most. Further, the SERS effect is distance sensitive. The increased electrical field is greatest on the metal surface and decreases exponentially with distance from the surface. As a consequence, parts of the adsorbate that are * To whom correspondence should be addressed: School of Chemistry, P.O. Box 23, Monash University, Clayton, Victoria 3800, Australia. Phone: +61 3 9905 4525. Fax: +61 3 9905 4597. E-mail: [email protected].

located close to the surface give rise to more enhanced Raman bands than parts that are directed away from the surface. In the chemical model1,3,4 the metal forms a charge transfer complex with the adsorbate by transferring an electron from the Fermi level into an empty π* orbital of the adsorbate. Such a charge transfer transition has large oscillator strength and therefore results in a large increase in polarizability of the molecule at the surface. If the incident light is in resonance with the charge transfer band, strongly enhanced Raman scattering occurs. The most common surface substrates used in SERS include colloids, electrochemically roughened electrodes, and vacuum deposited island films of Ag, Au, and Cu. The SERS of nucleic acid bases (NAB) on silver colloids and electrodes have been studied extensively;12-24 however, many of the reports contradict each other with regard to the interaction between the base and the surface. For example, adenine (Figure 1) has been reported to adsorb to Ag electrodes with both a flat14,16,18 and erect13,17 orientation with the interaction reported via the N7 nitrogen,13,17 the N1 nitrogen,19 the external amino group,12,13 or the purine ring.14,18,20 Some of these contradictions are due to unreliable assignments of Raman bands20 based on group theory, semiempirical calculations, or comparisons with spectra from related compounds involving isotopes. We have now reinvestigated the SERS of adenine on a range of substrates and carried out isotopic substitutions and DFT calculations in order to resolve these contradictions. Therefore, in the first part of this study we report the results of density functional theory (DFT) calculations (B3LYP/ 6-31++G(d,p)) to assign the Raman spectra of the purine base adenine and three different deuterated isotopes of adenine. Semiempirical25-28 and ab initio29-34 calculations of the vibrational spectra of adenine have been reported. The latter were performed at the Hartree-Fock (HF) level of theory using STO3G,29 3-21G,31 4-21G,30 and 6-31G(d,p)32,33 basis sets. Since HF calculations do not take electron correlation into consideration, predicted wavenumber values have to be scaled with general scaling factors or least-squares fit methods to adjust to

10.1021/jp010789f CCC: $22.00 © 2002 American Chemical Society Published on Web 12/13/2001

102 J. Phys. Chem. B, Vol. 106, No. 1, 2002

Figure 1. Structure of adenine and three of its deuterated isotopomers.

observed experimental wavenumber values. DFT methods take electron correlation into account and no scaling is necessary. Recently, DFT BP/6-311G calculations of the vibrational spectra of the NABs adenine, cytosine, guanine, and thymine have been published.35 And the matrix isolation FTIR spectrum of adenine has been assigned on the basis of B3LYP/6-31G(d,p) calculations.34 We have used a slightly different approach for the assignment of the Raman spectrum of polycrystalline adenine using a larger basis set augmented by d and p polarization functions as well as diffuse functions (6-31G++(d,p)). In the second part of this study we report the SERS spectra of adenine and its deuterated isotopes on different silver surface substrates. On the basis of our theoretical calculations and the experimental SERS spectra, we suggest models for the interaction of the bases with the different surface substrates. These models are based on the application of specific surface selection rules.9 A subsequent paper will address the influence of pH, electrode potential, and excitation wavelength to obtain a further insight into the adsorbate-metal interaction. 2. Experimental Section Adenine was purchased from Sigma-Aldrich. The reagents used were of analytical reagent grade. All glassware was cleaned with aqua regia followed by extensive rinsing with water prior to use. Aqueous solutions were prepared using 18 MΩ high purity water. Deuteration was performed using D2O (99.9%, Aldrich) and different deuterated isotopes were produced utilizing the differing reactivities of the hydrogens in adenine.36 Dissolution of adenine in D2O at room temperature resulted in adenine-d3, in which all nitrogen bound hydrogens are exchanged by deuterium. This product was purified by two subsequent recrystallizations in D2O at room temperature. Upon heating an adenine-d3 solution in D2O to 80 °C for 24 h, the C8 bound hydrogen was also exchanged (adenine-d4). Dissolution of adenine-d4 in H2O at room temperature left only C8 deuterated, producing adenine-d1. This product also had to be purified by 2-fold recrystallization in H2O at room temperature. Complete deuteration at C8 was confirmed by the disappearance of the singlet at 8.14 ppm in the H NMR spectra of adenine-d1 and adenine-d4. The singlet at 8.20 ppm due to the C2 bound hydrogen remains unchanged for all adenine derivatives. The nitrogen bound hydrogens could not be monitored by H NMR. Isotope exchange of these hydrogens was monitored by the shift of the symmetric NH2 stretching band from 3127 to 2320 cm-1 in the FTIR spectrum. The silver electrode was roughened in situ in a spectroelectrochemical cell (Figure 2) consisting of a three-electrode

Giese and McNaughton

Figure 2. Spectroelectrochemical cell used in SERS experiments at a silver electrode.

arrangement embedded in Teflon, with the silver electrode as the working electrode, a platinum counter electrode, and Ag/ AgCl reference electrode. Prior to the roughening process the working electrode was thoroughly prepared using the following procedure and then placed into the spectroelectrochemical cell: (1) polish with SiC paper (P#4000) and then immerse in aqua regia for 1 min; (2) rinse with water, re-polish, and rinse again with water; (3) degrease with acetone, sonicate in water for 15 min and rinse again with water. The working electrode was roughened by applying oxidation and reduction cycles (ORC) with 0.1 M KCl (degassed) as the electrolyte. The potential was changed from -400 mV to +400 mV (relative to the standard Ag/AgCl electrode) and back to -400 mV at a sweep rate of 200 mV s-1. This ORC was repeated three times. After the roughening process the electrolyte was replaced with a 10-4 M sample solution. SERS spectra were recorded under open circuit conditions by focusing the laser on the roughened electrode through an optical quartz window. Ag colloid was prepared according to the standard procedure of Lee and Meisel.37 AgNO3 (90 mg) was dissolved in 500 mL of water and heated to boiling with extensive stirring. A 10 mL portion of 1% aqueous trisodium citrate was added dropwise, and the reaction mixture was boiled for another 90 min. After cooling the mixture to room temperature, the volume was adjusted with water to 500 mL. The resultant colloid was yellowish gray with an absorption maximum at 420 nm. Colloidal suspensions were stable for several weeks and were stored in the dark at 5 °C. For SERS measurements, 10 µL sample solutions were mixed with 10 µL of colloidal suspension to give a final sample concentration of 10-4 mol dm-3. The samples were placed in a 3 mm dia. glass capillary for the recording of SERS spectra. Silver island films were prepared by the vacuum deposition of silver onto a microscope slide at room temperature. Film thickness was 80 Å. A 5 µL sample (10-4 M) was deposited on the island film and the solvent evaporated in a desiccator at room temperature and 1 atm. Excess sample was washed off by extensive rinsing with water. Finally, the island film was dried in a vacuum desiccator. Raman and SERS spectra were recorded on a Renishaw Raman microspectrometer system 2000 equipped with a HeNe laser (λ ) 632 nm) and an electrically cooled CCD detector. The laser power at the sample was 4 mW. Exposure time of the CCD was 10 s, and 10 scans were coadded. For SERS measurements at the silver island film, the laser power was reduced to 0.04 mW to avoid photodecomposition of the sample. To compensate for the reduced laser power, 100 scans were accumulated. Even at low laser power, decomposition could not

Adenine Adsorption to Silver Surfaces

J. Phys. Chem. B, Vol. 106, No. 1, 2002 103

Figure 3. Theoretical (a,c,e,g) and experimental (b,d,f,h) NRS spectra of polycrystalline adenine (a,b), adenine-d1 (c,d), adenine-d3 (e,f), and adenine-d4 (g,h).

be prevented completely, as evidenced by the appearance of a band at 1586 cm-1 due to graphitic carbon. DFT calculations were carried out using Gaussian 98 software.38 All calculations were performed by applying the hybrid of Becke’s nonlocal three parameter exchange and correlation functional and the Lee-Yang-Parr correlation functional (B3LYP). The 6-31++G(d,p) split valence-shell basis set augmented by d polarization functions on heavy atoms and p polarization functions on hydrogen atoms as well as diffuse functions for both hydrogen and heavy atoms was used.

3. Results and Discussion Density Functional Theory Calculations and Assignment of the Normal Raman Spectrum. The geometry of adenine (Figure 1) was fully optimized at the B3LYP level of theory using the 6-31++G(d,p) basis set without any constraint on the planarity. This was followed by a calculation of the harmonic vibrational wavenumber values and relative Raman intensities at the same level of theory and using the same basis set. In initial calculations assuming a planar geometry (Cs symmetry)



TABLE 1: Assignment of Theoretical Wavenumber Values to Experimental Bands in the Normal Raman Spectrum of Adenine-d0 and Description of the Different Modes wavenumber/cm-1 Santamaria et mode

exp.a

exp.b

al.35

Nowak et al.34

this study

BP/6-311G exp.c B3LYP/6-31G(d,p) exp.d B3LYP/6-312+G(d,p) plane

1 2

1673 1675 1604/1638 1597f

1634 1552

1639 1612

1641 1617

1674 1613

1665 1643

3 4

1508f 1482

1612f 1482/1462

1583 1450e

1599 1482

1584 1502

1597 1483

1613 1524

5

1421f

1418f

1421e

1474

1487

1463

1510

6

1371f

1387e

1419

1416

1419

1441

7

1335f

1370

1355e

1389

1400

1372

1423

8

1309f

1331

1328

1345

1350

1333

1372

1307f

1292

1328

1342

1333

1365

9 10

1253f

1248f

1260

1290

1317

1308

1341

11

1157/1234 1235f

1213

1240

1250

1248

1272

12

1164f

1202

1229

1228

1234

1246

13

1126f

14 15 16 17 18

913f 849f 951

19 20

797

21

724

22

660/684

1126f

1103

1127f

1129

1162

1148

1023f

1053

1032f

1065

1126

1085

940f

969 917 914 853

1005 958 927 887

1000 953 925 882

1025 942 899

1012 974 942 899

782 807

848 802

831 793

839 797

849 805

693

717

713

723

726

898f 848

722

674

23 24

622

620

25

658

600

610

607

623

618

640

566

568

560

576

528 521

536

540 530

514

536

520

28

530

499e

29 30

337

511 296

327 249

240

666

655

543

31 32 33 34

685

649

26 27

535

672

553 514

261 209 161

513

504 298 276 242 214

269 244 205 162

518 300 330

277 220 167 133

assignmentg

in in

sciss NH2, str C6-N10, C5-C6 str N3-C4, N1-C6, C5-N7, N7-C8, bend N9-H in sciss NH2 in str N7-C8, bend C8-H, sciss NH2 in str C2-N3, N1-C6, bend C2-H, sciss NH2 in str C4-N9, C4-C5, C6-N10, N7-C8, bend C2-H in bend C2-H, N9-H, str C8-N9, C4-N9 in bend C2-H, C8-H, N9-H, str C6-N1, C8-N9, N3-C4 in str C5-N7, N1-C2, bend C2-H, C8-H, in str C2-N3, N1-C2, C5-C6, C5-N7 in bend C8-H, N9-H str N7-C8 in rock NH2, str C5-N7, N1-C2, C2-N3 in bend C8H, N10-H11, str C4-N9, N3-C4, C6-N10 in str C8-N9, A bend N9-H, C8-H, in rock NH2 out wag C2-H in def R5 (sqz group N7-C8-N9) in def R6 (sqz group N1-C2-N3), R5 (str C5-N7) out wag C8-H out def R6 (wag C4-C5-C6), wag C8-H in ring breath whole molecule (distorted) out def R5, R6 (tors C4-C5-C6, wag N3-C4-N9) out def R5 (wag C5-N7-C8), wag C8-H, N9-H, in def R6 (sqz group C4-C5-C6, N1-C6-N10), R5 (sqz group C5-N7-C8) out wag C2-H, N9-H out tors NH2 in/out def R6 (sqz group N1-C6-C5, C2-N3-C4) in/out wag N9-H, def R6, R5 (sqz group N3-C4-N9) out wag N9-H, tors NH2 out def R6, R5 (wag N1-C2-N3-C4, C5-N7-C8-N9), wag C6-N10 in bend C6-NH2 out butterfly out wag NH2, def R out tors molecule, wag C6-NH2

IR and Raman combined results of adenine in crystalline powder, from Hirakawa et al.51 b IR and Raman wavenumber values of polycrystalline adenine, from Majoube.26 c IR wavenumber values of matrix isolated adenine in nitrogen and argon matrix, from Nowak et al.34 d Raman wavenumber values of polycrystalline adenine. e Denotes considerably different description of the mode. f Denotes different assignment of the mode to an experimental vibrational band. g Bend, bending; breath, breathing; def, deformation; rock, rocking; sciss, scissoring; str, stretching; wag, wagging; R5, five-membered ring; R6, six-membered ring. a

an imaginary wavenumber value for an out-of-plane vibration of the amino group was obtained, indicating that the totally planar geometry does not represent an energy minimum, but rather a transition state. Without any constraint on the planarity, the optimized geometry is nonplanar with the two hydrogens of the amino group located slightly above the plane of the purine ring (C1 symmetry). Since all wavenumber values are real, it can be deduced that the optimized structure represents an energy minimum. Low level Hartree-Fock calculations using small basis sets such as STO-3G or 4-21G predict a planar

geometry for adenine,29,30 but higher level calculations (HF and MP2) using the larger basis set, 6-31G(d) give the same result as our calculations, predicting adenine to be nonplanar in its electronic ground state.39 The DFT predicted spectrum of adenine agrees well with the experimental normal Raman spectrum (NRS) of polycrystalline adenine (Figure 3a,b). The calculated wavenumber values in the 1700 cm-1 to 1100 cm-1 region are overestimated by 2-4%, while there is less deviation below 1100 cm-1. The mean deviation of the predicted wavenumber values from the experi-



TABLE 2: Theoretical and Experimental Wavenumber Values of Adenine-d1 as a Polycrystalline Powder (NRS) and Adsorbed to a Silver Colloid (SERS) wavenumber value/cm-1 mode B3LYP/6-312+G(d,p) NRS SERS colloid 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1665 1640 1613 1510 1488 1437 1421 1365 1352 1336 1248 1185 1112 1011 974 954 917 864 810 731 723 684 617 606 562 539 529 514 510 298 275 213 167 106

1673 1610 1596

1544? 1504?

1456

1439

1404 1362 1323 1309 1233 1173 1153 1022

1376 1376 1335 1309 1252 1151? 1151 1028

955 918 859 798

951

718 620

886 793 732 686 629 603

529 529

549 549

318

323

plane

assignmenta

in in in in in in in in in in in in in in out in in in out out in out in out out out in in/out out out in out out out

sciss NH2, str C6-N10, C5-C6 str N3-C4, N1-C6, C5-N7, bend N9-H sciss NH2 sciss NH2, Bend C2-H, str C6-N10, C6-N1, C2-N3 str N7-C8, C2-N3, C4-C5, bend C8-D str C4-N9, C4-C5, C6-N10, N7-C8, bend C2-H, sciss NH2 bend C2-H, N9-H, (str C8-N9, N1-C2) bend C2-H, N9-H, N10-H12, str C5-N7, C6-N10, N1-C2 str N1-C2, C6-N1, C4-C5, N3-C4, C8-N9, bend N9-H str C2-N3, C5-N7, bend N9-H rock NH2, str C5-N7, N1-C2, C2-N3 str C6-N10, N3-C4, C4-N9, bend N9-H, C2-H, N10-H11, C8-D str C8-N9, bend N9-H rock NH2 wag C2-H def R5 (sqz group N7-C8-N9), R6 (sqz group N1-C2-N3), bend C8-D def R5 (sqz group N7-C8-N9), R6 (sqz group N1-C2-N3), bend N9-H, C8-D bend C8-D, def R6 (sqz group N1-C2-N3) def R6 (wag C4-C5-C6, N1-C2-N3) def R5 (wag N7-C8-N9), wag C8-D ring breath whole molecule (distorted) def R6 (tors C4-C5-C6) def R5 (sqz group C5-N7-C8), R6 (sqz group C4-C5-C6) wag C8-D, def R6 (wag N1-C2-N3), R5 (tors C5-N7) wag N9-H, C8-D tors NH2 def R6 (sqz group N1-C6-C5, C2-N3-C4) def R6, R5 (sqz group N3-C4-N9, N1-C6-N10, C6-C5-N7), wag N9-H wag N9-H, C8-D, tors NH2, def R def R6, R5 (wag N1-C2-N3-C4, C5-N7-C8-N9), wag C6-N10 bend C6-NH2 butterfly wag NH2 wag C6-NH2

a Bend, bending; breath, breathing; def, deformation; rock, rocking; sciss, scissoring; str, stretching; wag, wagging; R5, five-membered ring; R6, six-membered ring.

mental data over the whole wavenumber region is less than 2.5%. Although the relative intensities are not correctly predicted for all Raman bands, they still provide some useful help for the assignment of the normal modes in the Raman spectrum. The motions that contribute the most to the different normal modes according to the DFT calculations are listed in Table 1. To assist in gaining a reliable assignment, the vibrational spectra of three differentially deuterated isotopomers were also investigated and the theoretical and experimental spectra of these isotopomers are also illustrated in Figure 3. Normal mode descriptions are presented in Tables 2-4. The modes in all tables are numbered according to decreasing wavenumber values calculated at the B3LYP/6-31++G(d,p) level. Due to isotope shifts, corresponding modes in the spectra of the different adenine analogues may have different mode numbers. In this discussion we focus on those bands where current assignments are ambiguous or those that we consider the most important in the context of the SERS. We also compare our assignments with the highest levels of theoretical prediction published by other authors so far. Nowak et al. report calculations at the B3LYP/6-31G(d,p) level34 and assign harmonic vibrations to the matrix isolation FTIR spectrum of adenine. Santamaria et al. recently published theoretical predictions on the basis of BP/6-311G calculations.35 None of these studies considered deuterated isotopomers. The vast majority of vibrations are complex and involve strongly coupled motions. Contributions from scissoring of the amino group reach from 1674 cm-1 down to 1463 cm-1. While the Raman bands at 1674 and 1597 cm-1 are dominated by this

scissoring motion, the vibration at 1613 cm-1 is due almost exclusively to skeletal stretching within the purine ring without any contribution from the external amino group. The former are expected to be strongly affected by deuteration of the amino group. Indeed, the first two bands are basically not altered in the NRS of adenine-d1 but are red shifted significantly in adenine-d3 and adenine-d4. In the NRS of these isotopomers, new vibrations dominated by ND2 scissoring appear in the 1100 cm-1 to 1200 cm-1 region. The adenine-d3 Raman band at 1607 cm-1 does not correspond to the band at 1613 cm-1 in the spectrum of adenine-d0, but is due rather to a vibration similar to mode 1 at 1674 cm-1. It involves similar ring stretching motions but lacks coupling to amino scissoring as a result of deuteration. Although mode 2 at 1613 cm-1 does not contain any substantial involvement from the amino group, it is considerably red shifted upon N deuteration. The experimentally observed shift is almost triple the predicted shift (-44 cm-1 compared to -15 cm-1). The relative Raman intensity is predicted well only for the band at 1597 cm-1 but not for those at 1613 and 1674 cm-1, which are weaker than predicted. The band at 1674 cm-1 is particularly weak, and it can hardly be distinguished from the baseline. Its presence and assignment, however, are supported by a very strong absorption band at 1672 cm-1 in the FTIR spectrum of adenine (data not shown). Our assignments in the 1450 cm-1 to 1700 cm-1 region do not agree with the original assignments of Nowak et al. derived on the basis of HF/6-31G(d,p) calculations.33 The same authors, however, later corrected their assignments in their B3LYP/ 6-31G(d,p) study, which produced essentially the same results



TABLE 3: Theoretical and Experimental Wavenumber Values of Adenine-d3 as a Polycrystalline Powder (NRS) and Adsorbed to a Silver Colloid (SERS) wavenumber value/cm-1 mode

B3LYP/6-31G++(d,p)

NRS

1 2 3 4 5 6 7 8 9 10

1645 1628 1534 1510 1443 1394 1374 1347 1333 1245

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1214 1188 1097 974 944 901 868 851 849 806 704 683 656 595 561 522 487 405 385 298 251 212 158 82

1607 1569 1515 1466 1422 1367 1367 1332 1305 1250 1226 1178 1152 1104 968 938 886

SERS colloid 1525 1464 1442 1373 1345 1324 1267 1234 1190 1107 978 950

859

867

797 709

789 721 689

647 602 559 525 511

604 566 537

303

300

plane

assignmenta

in in in in in in in in in in

str C5-C6, C6-N10, C4-C5, C2-N3, N1-C2 str N3-C4, N1-C6, C5-N7, N7-C8 str C6-N10, N7-C8, C2-N3, N3-C4, sciss ND2, bend C2-H, C8-H str N7-C8, C2-N3, N1-C6, bend C8-H-C2-H str C4-N9, C4-C5, C6-N10, N7-C8, bend C2-H, sciss ND2 bend C2-H, str C4-N9, N1-C6 str C5-N7, N1-C2, C4-C5, C6-N10, sciss ND2 bend C8-H, str C2-N3, C8-N9, C5-C6, N1-C2 str C8-N9, C5-N7, N1-C2, bend C8-H str C8-N9, bend C8-H, C2-H, N9-D

in in in out in in in in out out in out out in out in in out out out in out out out

def R6 (str C6-N1, N1-C2, C2-N3), R5 (str C5-N7), bend N10-H12 sciss ND2, bend C8-H sciss ND2, str C6-N10, C4-N9, bend C8-H wag C2-H def R5 (sqz group N7-C8-N9) def R6 (sqz group N1-C2-N3), R5 (str C5-N7), A bend C8-H, N9-D bend N9-D, rock ND2 rock ND2, bend N9-D wag C8-H def R6 (wag C4-C5-C6, N1-C2-N3), wag C8-H ring breath whole molecule (distorted) def R5, R6 (tors C4-C5-C6, wag N3-C4-N9) def R5 (wag C5-N7-C8, C4-N9-C8) def R6 (sqz group C4-C5-C6), R5 (C5-N7-C8) def R6 (wag N1-C2-N3), R5 (tors C8-N9) def R6 (sqz group N1-C6-C5, C2-N3-C4) def R5, R6 (sqz group N3-C4-N9, C6-C5-N7, N1-C6-N10) wag N9-D, tors ND2 tors ND2, wag N9-D def R6, R5 (wag N1-C2-N3-C4, C5-N7-C8-N9), wag C6-N10 bend C6-ND2, def R5, R6 (sqz group C6-C5-N7) butterfly def R5, R6 wag ND2


to our predictions.34 The only difference between their assignment and ours is the description of mode 4. In contrast to us, Nowak et al. 34 found no substantial contribution of NH2 scissoring for mode 4. The predicted and experimentally confirmed moderate red shifts of this mode upon deuteration at the amino group (-17 cm-1) support our finding that it contains some contribution of NH2 scissoring. In contrast to Santamaria et al.,35 but in agreement with Nowak et al.,34 we did not find any significant involvement of the amino group in mode 2. All calculations, however, agree in that the vibration at 1597 cm-1 is an almost pure amino scissoring motion without any contribution from the purine ring. There has been controversy about a low intensity vibrational band at 1505 cm-1. While some authors consider it to be a fundamental,40,41 others do not.34,42 Our assignments support the assumption that it does not arise from a fundamental. Between 1450 and 1000 cm-1, bending of the external NH and CH angles becomes important. These motions are coupled to stretching and deformation motions of both the pyrimidine and imidazole rings. Rocking of the amino group, which again is coupled to purine ring motions, can also be found in this region. Modes 11 and 12 are predicted at 1272 and 1246 cm-1 respectively with similar medium intensity. Instead, only one medium band at 1248 cm-1 can be found in the experimental spectrum with a small shoulder to the red. In contrast to mode 11, mode 12 is predicted to contain considerable contribution from NH2 rocking. It is, therefore, expected to be unaffected by deuteration at C8 but to red shift upon deuteration of the

amino group. This predicted trend is best matched when the experimental band at 1234 cm-1 is assigned to mode 12, the intensity of which is overestimated considerably in the theoretical Raman spectrum of adenine-d0. Mode 11, experimentally observed at 1248 cm-1 in the adenine-d0 spectrum, does not have an equivalent band in the spectra of the deuterated isotopomers. This assignment of modes 11 and 12 is also supported by FTIR data (not shown). Mode 11 is predicted to be three times more intense in the IR than mode 12. Indeed, the experimental FTIR spectrum shows an intense band at 1251 cm-1 (corresponding to 1272 cm-1 in the theoretical spectrum and 1248 cm-1 in the experimental Raman spectrum), with a shoulder to the red. Mode 15 is an almost pure NH2 rocking motion. It shifts from 1025 cm-1 in the spectra of adenine-d0 and adenine-d1 to 852 cm-1 in the spectra of adenine-d3 and adenine-d4. Santamaria et al.35 used their calculations to assign the vibrational spectra reported by various experimental groups, and their assignments of the different experimental spectra are somewhat contradictory. Still, none of their assignments of modes 9-17 agrees with our assignment, but are generally shifted by one band. For instance, in our assignment the Raman band at 1022 cm-1 corresponds to mode 15 predicted at 1020 cm-1, whereas Santamaria et al. assign this band to mode 14, which they predict at 1053 cm-1. In addition, our descriptions of the modes 4-7 differ quite markedly form those reported by Santamaria et al.35 Agreement with the assignment of Nowak et al.34 is much better, although some discrepancies persist.



TABLE 4: Theoretical and Experimental Wavenumber Values of Adenine-d4 as a Polycrystalline Powder (NRS) and Adsorbed to a Silver Colloid (SERS) wavenumber value/cm-1 mode

B3LYP/6-312+G(d,p)

NRS

SERS colloid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1644 1626 1528 1483 1442 1391 1371 1343 1290 1212 1209 1114 987 974 940 876 853 822 809 725 701 682 604 594 535 520 482 405 385 296 249 206 157 84

1605 1569 1508 1447 1404 1371 1361 1324 1283 1182

1541 1518 1445 1403 1370 1334 1311 1239 1196

1116 1014

1109 1109?

939 892 852 796

936

731 707 618 602

849 790 719 682 604

526 509

537

301

292

plane

assignmenta

in in in in in in in in in in in in in out in in in in out out in out out in out in in out out out in out out out

str C5-C6, C4-C5, C6-N10, C2-N3, N1-C2 str N3-C4, N1-C6, C5-N7 str C6-N10, C2-N3, C6-N1, bend C2-H str N7-C8, C2-N3, bend C2-H, C8-D str C4-N9, C6-N10, C4-C5, bend C2-H bend C2-H, str C4-N9, N1-C6 str C5-N7, N1-C2, C4-C5, C6-N10 str C2-N3, N1-C2, C6-N1, C5-C6 str C8-N9, C5-N7, bend N9-D, C8-D, C2-H str C5-N7, C8-N9, N1-C2, C2-N3, (sciss ND2) sciss ND2, bend C2-H sciss ND2 bend C8-D, N9-D (str C6-N10, def R6 (sqz groups N3-C4-C5, N1-C2-N3)) wag C2-H def R5 (sqz group N7-C8-N9) def R6 (sqz group N1-C2-N3), R5 (sqz group C8-N9-C4), bend C8-D, N9-D rock ND2, (str C6-N1, N1-C2) a bend C8-D, N9-D def R6 (wag C4-C5-C6, N1-C2-N3) def R5 (wag N7-C8-N9), wag C8-D ring breath whole molecule (distorted) def R5, R6 (tors C4-C5-C6, tors C6-N1, wag N3-C4-N9, C5-N7-C8) wag C8-D, C2-H, def R6 (wag N1-C2-N3), R5 (tors C5-N7) def R6 (sqz group N3-C4-C5), R5 (sqz group C5-N7-C8) wag C8-D, C2-H def R6 (sqz group C5-C6-N1, C2-N3-C4) def R5, R6 (sqz group N3-C4-N9, C6-C5-N7, N1-C6-N10) wag N9-D, tors ND2 tors ND2, wag N9-D def R6, R5 (wag N1-C2-N3-C4, C5-N7-C8-N9), wag C6-N10 bend C6-ND2 butterfly def R5, R6, wag C6-ND2 wag ND2


While we assign the bands at 1162, 1126, and 1025 cm-1 to modes 13-15, Nowak et al. do not consider the former band to correspond to a fundamental and assign the latter two bands to modes 13 and 14. These different assignments are probably not due to differences in calculations, but rather to the different experimental methodology. Matrix isolation, as employed by Nowak et al.,34 leads to the presence of extra bands arising from matrix effects, which partially complicate spectral assignment. Our assignments are supported by the relative Raman intensities of modes 9-17 that were predicted reasonably accurately by our calculations (with the exception of mode 12, vide supra), and by the good correlation between predicted and observed shifts upon deuteration. Unfortunately, neither of the other authors report Raman intensities, and thus no comparison can be made on this basis. The low wavenumber region between 1000 and 200 cm-1 consists of both in-plane and out-of-plane vibrations. Most of the out-of-plane modes are too weak to be observed in the experimental Raman spectrum. In fact, the only out-of-plane vibrations that can be identified in the experimental spectrum are the ones giving rise to the weak bands at 839, 797, and 560 cm-1. These vibrations involve mainly wagging of the C2-H, C8-H, and N9-H bonds, and only the vibration at 797 cm-1 contains significant contributions from an out-of-plane deformation of the purine ring. The more intense band at 536 cm-1 has also got some out-of-plane character, but it derives most of its intensity from an in-plane deformation of the internal angles of the pyrimidine ring. The very intense ring breathing mode is also in this wavenumber range. In this vibration, all bonds of

the purine ring expand and contract in phase. However, the vibration at 723 cm-1 is not a pure ring breathing mode, but is markedly distorted. Our assignment for Raman bands at wavenumber values smaller than 850 cm-1 agrees well with the ab initio and DFT results published previously. In general, our B3LYP/6-31++G(d,p) calculations confirmed the normal coordinate analysis by Nowak et al.34 at the B3LYP/6-31G(d,p) level of theory. The inclusion of diffuse functions did not lead to substantial differences in the predicted spectrum. Surface-enhanced Raman Spectroscopy on Silver Surfaces. Surface-enhanced Raman spectra of adenine at neutral pH were obtained using three different silver substrates: a citrate reduced silver colloid, an electrochemically roughened silver electrode, and a vacuum deposited silver island film (Figure 4). In the SERS spectra, several bands are shifted by up to 20 cm-1 compared to their corresponding wavenumber values in the normal Raman spectrum of the polycrystalline solid. These shifts are most likely due to the interaction of adenine with the metal surfaces, rather than the different states of aggregation. Unfortunately, adenine is insufficiently soluble in water at neutral pH to obtain an NRS spectrum in solution. However, the NRS spectrum of the polycrystalline Ag+-adenine complex, prepared by mixing aqueous AgNO3 and adenine solutions and subsequent solvent evaporation, exhibits essentially the same shifts as the SERS spectra compared to the NRS spectrum of polycrystalline adenine. The observed shifts between the SERS spectra and the NRS spectrum of adenine in Figure 4 are to be expected if there is a change of electron distribution within the adenine molecule


Giese and McNaughton the ring bond strength is weakened as a result of coordination between the electron donating nitrogens and the electronaccepting metal, causing those vibrational modes in the higher wavenumber region that are dominated by stretching motions to shift to lower wavenumber values. Models for the Interaction of Adenine with the Silver Surfaces. The combination of DFT calculations and the deuteration study has allowed us to assign all bands in the SERS spectrum of adenine in a silver sol with reasonable reliability. The SERS spectra obtained with a silver electrode and a silver island film have been interpreted in an analogous manner, leading to the assignments listed in Table 2. This knowledge can then be used to deduce information about the interaction of adenine with the metal surfaces in the different systems. Specific surface selection rules have been derived from the electromagnetic enhancement theory that can be employed to interpret SERS spectra with regard to adsorbate-surface interaction.9,45 Vibrations that derive their Raman intensity from a change in polarizability perpendicular to the surface are expected to be preferentially enhanced. Further, the distance dependence of the electromagnetic enhancement causes vibrational modes of a moiety located close to the surface to be enhanced more than modes of a moiety directed toward the bulk solution. To allow a better comparison of the enhancement of the different modes in the SERS spectra, relative enhancement factors (rel. EF) are listed in Table 5 and illustrated in Figure 6. They are normalized to the ring breathing mode (rb) at about 730 cm-1 and are defined as

rel. EF(νj) ) Figure 4. NRS spectrum of adenine (a) and the SERS spectra of adenine adsorbed at a silver colloid (b), a silver electrode (c), and a silver island film (d).

as a result of the formation of a charge transfer complex at the metal surface. Chemical enhancement, therefore, is expected to contribute to the overall enhancement of the spectra in addition to electromagnetic enhancement. Differences between the SERS spectra on the three substrates both in wavenumber values and in relative intensities can be explained by the different contributions of the two enhancement mechanisms. These spectral differences may also be explained by the possible different orientations of the adsorbed adenine relative to the metal surface depending on the substrate. The assignment of the SERS bands given in Table 5 was confirmed by comparison with the SERS spectra of the three different deuterated analogues (Figure 5) and their shifts upon deuteration in the NRS spectra discussed above (Figure 3). We thus have a greater level of confidence in our assignment by carrying out these SERS studies on the deuterated species. It is interesting to note that those SERS bands that are shifted considerably compared to the respective NRS bands as a result of the interaction of adenine with the silver surface can be divided into two groups. The modes with wavenumber values higher than 1350 cm-1 are red shifted, while the modes with lower wavenumber values are blue shifted. The same trends have been observed in the SERS spectra of 1,10-phenanthroline43 and benzo[c]cinnoline.44 They have been attributed to the interaction of Ag with the heterocyclic nitrogens.43,44 The Ag molecule force constants constrain the vibrational motions of the adsorbate, especially for vibrations in the low wavenumber region dominated by ring bending deformations. Consequently, these vibrational modes are blue shifted. On the other hand,

Iνj,SERS‚Irb,NRS Iνj,NRS‚Irb,SERS

In all three Ag/adenine systems investigated, the rel. EF indicate contributions from N7 and/or the external amino group for most of the preferentially enhanced bands, i.e., the NRS bands at 1483, 1419, 1333, 1234, 1025, 942, 623, and 330 cm-1 are considerably enhanced. In contrast, most of the vibrations involving N1 and N3 motion do not appear in the SERS spectra. This suggests that interaction with the metal surface takes place via both N7 and the amino group. This conclusion is in agreement with the results by Watanabe17 and Otto et al.13 The former17 concluded end-on adsorption of adenine to a silver electrode via N7 by assigning the strongly enhanced band at 1335 cm-1 to the N7-C5 stretching mode. The latter13 pointed out that for steric reasons adenine-metal interaction via N7 is difficult without additional contact of the exocyclic amino group. They further supported this conclusion by showing that the preferentially enhanced bands at 325, 621, 1029, and 1194 cm-1 are all assignable to vibrations involving the amino group. Those assignments were all based on a semiempirical normal coordinate analysis by Tsuboi et al.46 Our DFT calculations confirmed contributions of the amino group to these four vibrations. It should be noted, however, that the band at 1194 cm-1 did not consistently appear in all of the SERS spectra of adenine we recorded on a silver electrode. Although the vibrations pointed out by Otto et al.12,13 are not those with the highest rel. EF values in our spectra, their proposed interaction via N7 and the external amino group is supported by our data. This is more obvious for the Ag colloid than for the Ag electrode studied by Otto et al. In the latter system we found unexpectedly high rel. EF values for vibrations dominated by motions of N9, especially for the NRS band at 1419 cm-1. Nevertheless, interaction of adenine via N9 is



TABLE 5: Assignment of the Normal Vibrational Modes of Adenine to the SERS Bands at Three Different Silver Substrates and Relative Enhancement Factors of These Modes wavenumber value/cm-1 NRS mode 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

SERS colloid

1674 1613 1597 1483 1463 1419 1372 1333 1333 1308 1248 1234 1162 1126 1025

rel. EF

electrode

0.0 1545? 1513? 1458

1516? 1460

island film

0.0

1586? 1540?

0.3

1457

1.9

1397 1370 1330 1330

1399 1372b 1326 1326

1268 1244 1190 1137 1025

0.8 0.7 0.4 0.4 0.0 0.1 0.3 0.0 0.1 0.5

961 894

0.4 0.1

958

0.6 0.0

790 733 690b

0.5 1.0 new

788 731 690

0.6 1.0 new

623

626

0.4

621

25 26 27 28 29 30

560

563

0.5

536

543

0.1

31 32 33 34

330

942 899 839 797 723

1397 1372 1336 1336

0.2

rel. EF

504

1270 1245 1117 1029

0.5

1276 1244 1137 1094 999 954

0.0 0.9 1.3 0.9 0.6 0.6 0.0 0.2 0.3 0.0 0.1 1.5 new 0.2 0.0

0.4

788 732 691 649 624

0.8 1.0 new new 0.5

553

0.7

559

1.4

536

0.1

0.0

0.0

0.0

0.0 326

0.8 0.8 0.0 0.0 0.8 0.0 0.1 0.5

rel. EF

325

0.5

326

0.5

plane in in in in in in in in in in in in in in in out in in out out in out out in out out in/out in/out out out in out out out

assignmenta sciss NH2, str C6-N10, C5-C6 str N3-C4, N1-C6, C5-N7, N7-C8, bend N9-H sciss NH2 str N7-C8, bend C8-H, sciss NH2 str C2-N3, N1-C6, bend C2-H, sciss NH2 str C4-N9, C4-C5, C6-N10, N7-C8, bend C2-H bend C2-H, N9-H, str C8-N9, C4-N9 bend C2-H, C8-H, N9-H, str C6-N1, C8-N9, N3-C4 str C5-N7, N1-C2, bend C2-H, C8-H, str C2-N3, N1-C2, C5-C6, C5-N7 bend C8-H, N9-H, str N7-C8 rock NH2, str C5-N7, N1-C2, C2-N3 bend C8H, N10-H11, str C4-N9, N3-C4, C6-N10 str C8-N9, A bend N9-H, C8-H, rock NH2 wag C2-H def R5 (sqz group N7-C8-N9) def R6 (sqz group N1-C2-N3), R5 (str C5-N7) wag C8-H def R6 (wag C4-C5-C6), wag C8-H ring breath whole molecule (distorted) def R5, R6 (tors C4-C5-C6, wag N3-C4-N9) def R5 (wag C5-N7-C8), wag C8-H, N9-H, def R6 (sqz group C4-C5-C6, N1-C6-N10), R5 (sqz group C5-N7-C8) wag C2-H, N9-H tors NH2 def R6 (sqz group N1-C6-C5, C2-N3-C4) wag N9-H, def R6, R5 (sqz group N3-C4-N9) wag N9-H, tors NH2 def R6, R5 (wag N1-C2-N3-C4, C5-N7-C8-N9), wag C6-N10 bend C6-NH2 butterfly wag NH2, def R tors molecule, wag C6-NH2

a Bend, bending; breath, breathing; def, deformation; rock, rocking; sciss, scissoring; str, stretching; wag, wagging; R5, five-membered ring; R6, six-membered ring. b Band appears in some, but not all spectra.

unlikely, since adenosine monophosphate (AMP), in which the N9 position is sterically blocked by the sugar phosphate moiety, gives a SERS spectrum, which is essentially identical to adenine.47 Considering the strong enhancement of most vibrations involving the amino group, it is surprising that the two bands in the NRS of adenine above 1500 cm-1 that are dominated by scissoring of the amino group, i.e., the bands at 1674 and 1597 cm-1, do not appear in the SERS spectrum of adenine adsorbed to a silver colloid or a silver electrode. The SERS bands at 1515 and 1543 cm-1 may correspond to these modes, but this assignment is precarious (vide supra). The intensities of these two bands are highly variable in the colloidal SERS spectrum, varying from absent to medium intensity comparable to the SERS band at 1460 cm-1. The band at 1585 cm-1 in the SERS spectrum of adenine adsorbed to a silver island film (Figure 4d) is probably due to graphitic carbon produced by the photodecomposition of adenine at the activated silver surface upon laser irradiation. 48,49 This band may obscure the presence of enhanced bands associated with the scissoring of the amino group. The particularly large enhancement of mode 15 at 1094 cm-1, dominated by rocking of the amino group, together with the relatively small enhancement of mode 17 at 954 cm-1, involving motions of N7 and N9, in the island film SERS spectrum may suggest interaction via the exocyclic amino group alone.

The rel. EF values in Figure 6 show that the out-of-plane vibrations are not preferentially enhanced compared to the strongly enhanced ring breathing mode and skeletal ring vibrations. In fact, the out-of-plane NRS bands at 839 and 504 cm-1 do not appear in any of the SERS spectra at all. This leads to the conclusion that in all three systems adenine is oriented in an upright position rather than adopting a flat orientation. Some bands due to out-of-plane vibrations do appear in all three SERS systems. Adenine, therefore, might adsorb to the surface substrates in a slightly tilted fashion. The relative intensities of these out of plane vibrations at about 790 and 560 cm-1 increase in the order colloid < electrode < island film. This indicates that the degree of tilt may increase in the same order. Our conclusion of a more or less upright orientation of adenine, particularly on the colloid surface, is in contrast with reports from Suh and Moskovits.20 These authors were the first to apply surface selection rules to the interpretation of the SERS spectrum of adenine. The absence of a SERS band corresponding to C-H stretching at about 3080 cm-1, supposedly a marker band for molecules standing up on the surface, led them to assume a flat orientation of adenine on a silver colloid. Their interpretation, however, depends on the assignment of the strongest SERS band at 735 cm-1 to a NH2 deformation rather than the ring breathing mode, another SERS marker band for


Figure 5. SERS spectra of adenine (a), adenine-d1 (b), adenine-d3 (c), and adenine-d4 (d) in a silver colloid.

Giese and McNaughton upright orientation. This assignment cannot be sustained, since ab initio and DFT calculations performed by us and others34,35 show no such NH2 deformation in the concerned wavenumber range. The closest vibrations in the NRS spectrum significantly involving the NH2 functional group are located at 559 and 1024 cm-1 and are unlikely to shift by 200-300 cm-1 upon adsorption. The absence of the C-H stretching band in the SERS spectrum may be due rather to the orientation of the adenine with the N7 moiety and the amino group directed toward the metal surface. In this case both the C-H bonds (C2-H and C8-H) are oriented practically parallel with respect to the metal surface. It should be noted here that the SERS spectrum of adenine adsorbed to a silver sol reported by Suh and Moskovits20 differs quite markedly from our equivalent spectrum with regard to several of the less intense SERS bands. The fact that those authors used a borohydride reduced silver colloid in contrast to our citrate reduced colloid cannot account for this discrepancy, since we obtained essentially identical SERS spectra with both types of colloids. The only difference observed was the larger overall enhancement achieved with the citrate reduced colloid (about 1 order of magnitude). Relatively poor reproducibility is a well-known problem of the SERS technique. Hence we decided to conduct every experiment at least 10 times and only include those features in our discussion that are consistent for all measurements. The spectra shown in the figures of this publication are typical spectra. Wherever applicable, considerable variations in band position and relative intensities are mentioned in the text. Kim et al.19 reported a reorientation of adenine adsorbed to a silver colloid on the basis of shifts in the ring breathing mode upon increasing the adenine concentration. According to these authors adenine adopts a flat orientation at low concentrations (∼10-6 M) and adsorbs in a more upright orientation, possibly

Figure 6. Relative enhancement factors of the different modes in the SERS spectra.

Adenine Adsorption to Silver Surfaces via N1, at higher concentrations (>9 × 10-6 M). While the logic of this reorientation model might be attractive given the surface area available to the adsorbate, spectral evidence is doubtful. A concentration dependence of the SERS spectrum of adenine has not been mentioned by other authors, and we did not observe any consistent variation of the SERS spectrum in the 10-6 M to 5 × 10-4 M concentration range. Further, in Kim et al.’s model19 the ring breathing mode is blue shifted when adenine adopts a flat orientation, in contrast to findings by Gao and Weaver.50 The latter authors reported a red shift of the ring breathing mode of monosubstituted benzenes for flat orientations due to the back-donation of electrons from the metal into antibonding π* orbitals. Additionally, Kim et al.’s postulate19 of interaction via N1 is based solely on the assumption of the highest negative charge density at that site rather than on spectroscopic evidence. Similar discrepancies exist for the interpretation of the SERS spectra of adenine on silver electrodes. Brabec and Niki18 assumed flat orientation by transferring results from their differential capacitance and electrocapillary studies of adenine adsorption at mercury electrodes to the adenine-silver system. The rel. EF values in Figure 6, however, are most consistent with the assumption of an upright, or at best tilted, orientation and interaction via N7 and the amino group. Further evidence of interaction via the nitrogen atoms rather than the ring π electron system is provided by the intense band at about 220 cm-1 in the SERS spectra of adenine on a silver colloid (Figure 4b) and a silver island film (Figure 4d). This band has been assigned previously to an Ag-N stretching vibration. In the electrode system (Figure 4c) this band is superimposed with the Ag-Cl band at 233 cm-1. 4. Conclusion In conclusion, the presented data suggest that adenine interacts with colloids, electrodes, and island films in similar but slightly distinctive fashions. In the colloidal system, adenine adsorbs essentially perpendicular to the surface via both N7 and the exocyclic amino group. On the electrode, adenine adopts a more tilted orientation while still interacting via N7 and the amino group. On the vacuum deposited island film, adenine also adsorbs in a tilted orientation, but with the interaction taking place mainly through the amino group. Our findings confirm Otto et al.’s results for SERS at a silver electrode. This, however, contradicts reports by several other authors, who proposed interaction via N1 or a flat orientation at both colloid and electrode. Of these studies, which concluded a flat orientation, comparison with the work by Suh and Moskovits is most noteworthy. Although we applied the same surface selection rules developed by these authors, we have come to contrasting conclusions. Their interpretation relied on the assignment of the strong SERS band at about 730 cm-1 to a NH2 deformation. Our high level DFT calculation clearly showed that such an assignment cannot be sustained, but that this band corresponds rather to the ring breathing mode, which is a marker band for an upright or at least tilted orientation. To gain further insight into the adenine metal interaction in these SERS systems the influence of pH, electrode potential, and laser excitation wavelength needs further investigation. Such an investigation is currently being undertaken in our group and will be published at a later date. References and Notes (1) Moskovits, M. ReV. Modern Phys. 1985, 57, 783. (2) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241.

J. Phys. Chem. B, Vol. 106, No. 1, 2002 111 (3) Otto, A.; Billmann, J.; Eickmans, J.; Ertuerk, U.; Pettenkofer, C. Surf. Sci. 1984, 138, 319. (4) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (5) Kneipp, K.; Wang, Y.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1996, 76, 2444. (6) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (7) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (8) Creighton, J. A. In Spectroscopy of Surfaces; Hester, R. J. H. C. a. R. E., Ed.; Wiley: New York, 1988; p 37. (9) Creighton, J. A. Surf. Sci. 1983, 124, 209. (10) Greslion, S.; Aigouy, L.; Boccara, A. C.; Rivoal, J. C.; Quelin, X.; Desmarest, C.; Gadenne, P.; Shubin, V. A.; Sarychev, A. K.; Shalaev, V. M. Phys. ReV. Lett. 1999, 82, 4520. (11) Markel, V. A.; Shalaev, V. M.; Zhang, P.; Huynh, W.; Tay, L.; Haslett, T. L.; Moskovits, M. Phys. ReV. B 1999, 59, 10903. (12) Otto, C.; Tweel, T. J. J. v. d.; Mul, F. F. M. d.; Greve, J. J. Raman Spectrosc. 1986, 17, 289. (13) Otto, C.; Mul, F. F. M. d.; Huizinga, A.; Greve, J. J. Phys. Chem. 1988, 92, 1239. (14) Koglin, E.; Sequaris, J. M.; Valenta, P. J. Mol. Struct. 1980, 60, 421. (15) Koglin, E.; Sequaris, J.-M.; Fritz, J.-C.; Valenta, P. J. Mol. Struct. 1984, 114, 219. (16) Itoh, K.; Minami, K.; Tsujino, T.; Kim, M. J. Phys. Chem. 1991, 95, 1339. (17) Watanabe, T.; Kawanami, O.; Katoh, H.; Honda, K.; Nishimura, Y.; Tsuboi, M. Surf. Sci. 1985, 158, 341. (18) Brabec, V.; Niki, K. Collect. Czech. Chem. Commun. 1986, 51, 167. (19) Kim, S. K.; Joo, T. H.; Suh, S. W.; Kim, M. S. J. Raman Spectrosc. 1986, 17, 381. (20) Suh, J. S.; Moskovits, M. J. Am. Chem. Soc. 1986, 108, 4711. (21) Camafeita, L. E.; Sanchez-Cortes, S.; Garcia-Ramos, J. V. J. Raman Spectrosc. 1996, 27, 533. (22) Sanchez-Cortes, S.; Garcia-Ramos, J. V. J. Raman Spectrosc. 1992, 23, 61. (23) Sanchez-Cortes, S.; Garcia-Ramos, J. V. Vib. Spectrosc. 1993, 4, 185. (24) Sanchez-Cortes, S.; Garcia-Ramos, J. V. Langmuir 2000, 16, 764. (25) Dhaouadi, Z.; Ghomi, M.; Austin, J. C.; Girling, R. B.; Hester, R. E.; Mojzes, P.; Chinsky, L.; Turpin, P. Y.; Coulombeau, C.; Jobic, H.; Tomkinson, J. J. Phys. Chem. 1993, 97, 1074. (26) Majoube, M. J. Mol. Struct. 1986, 145, 427. (27) Stepanian, S. G.; Sheina, G. G.; Radchenko, E. D.; Blagoi, Y. P. J. Mol. Struct. 1985, 131, 333. (28) Hrouda, V.; Florian, J.; Hobza, P. J. Phys. Chem. 1993, 97, 1542. (29) Florian. J. Mol. Struct. (THEOCHEM) 1992, 253, 83. (30) Wiorkiewicz-Kuczera, J.; Karplus, M. J. Am. Chem. Soc. 1990, 112, 5324. (31) Nowak, M. J.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta 1991, 47A, 87. (32) Nowak, M. J.; Rostkowska, H.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. J. Phys. Chem. 1994, 98, 2813. (33) Nowak, M. J.; Rostkowska, H.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta 1994, 50A, 1081. (34) Nowak, M. J.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. J. Phys. Chem. 1996, 100, 3527. (35) Santamaria, R.; Charro, E.; Zacarias, A.; Castro, M. J. Comput. Chem. 1999, 20, 511. (36) Elvidge, J. A.; Jones, J. R.; O’Brien, C.; Evans, E. A. J. Chem. Soc. D 1971, 394. (37) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ciolowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision 6; Gaussian, Inc.: Pittsburgh, PA, 1998. (39) Leszczynski, J. Int. J. Quantum Chem.: Quantum Biol. Symp. 1992, 19, 43. (40) Majoube, M. J. Raman Spectrosc. 1985, 16, 98-110. (41) Ten, G. N.; Nechaev, V. V.; Berezin, V. I.; Baranov, V. I. J. Struct. Chem. 1997, 38, 262.

112 J. Phys. Chem. B, Vol. 106, No. 1, 2002 (42) Majoube, M.; Millie, P.; Lagant, P.; Vergoten, G. J. Raman Spectrosc. 1994, 25, 821-836. (43) Muniz-Miranda. J. Phys. Chem. A 2000, 104, 7803. (44) Muniz-Miranda, M.; Sbrana, G. J. Raman Spectrosc. 1996, 27, 105. (45) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 5526. (46) Tsuboi, M.; Takahashi, S.; Harada, I. In Physicochemical Properties of Nucleic Acids; Duchesne, J., Ed.; Academic Press: New York, 1973. (47) Kneipp, K.; Kneipp, H.; Manoharan, R.; Hanlon, E. B.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Appl. Spectrosc. 1998, 52, 1493.

Giese and McNaughton (48) Cooney, R. P.; Mahoney, M. R.; Howard, M. W. Chem. Phys. Lett. 1980, 76, 448. (49) Mahoney, M. R.; Howard, M. W.; Cooney, R. P. Chem. Phys. Lett. 1980, 71, 59. (50) Gao, X.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858. (51) Hirakawa, A. Y.; Okada, H.; Sasagawa, S.; Tsuboi, M. J. Mol. Struct. 1985, 41, 209.

Surface-Enhanced Raman Spectroscopic and Density Functional

Recommend Documents