Genesis of Enhanced Raman Bands in SERS Spectra of 2

Article pubs.acs.org/JPCA

Genesis of Enhanced Raman Bands in SERS Spectra of 2‑Mercaptoimidazole: FTIR, Raman, DFT, and SERS Subhendu Chandra,† Joydeep Chowdhury,*,‡ Manash Ghosh,§ and G. B. Talapatra*,§ †

Department of Physics, Victoria Institution (College), 78 B, A. P. C. Road, Kolkata 700009, India Department of Physics, Sammilani Mahavidyalaya, Baghajatin Station, E. M. Bypass, Kolkata 700075, India § Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India ‡

S Supporting Information *

ABSTRACT: The surface enhanced Raman scattering (SERS) spectra of biologically and industrially significant, 2-mercaptoimidazole (2-MI) molecule have been investigated. The SERS spectra of the molecule at different concentrations of the adsorbate are compared with its Fourier transform infrared (FTIR) and normal Raman spectra (NRS) in varied environments. The optimized molecular structures and vibrational wavenumbers of the various forms (ca. cationic, neutral, ylidic, anionic) of the molecule have been estimated from the density functional theory (DFT). The vibrational signatures of the molecule have been assigned for the first time from the potential energy distributions (PEDs). The analyses of the Raman vibrational signatures reveal the coexistence of all the different forms of the molecule in the solid state and in aqueous solution. Concentration dependent SERS spectra of the molecule at neutral pH of the medium together with the multivariate data analyses techniques also suggest the concomitance of all the probable forms of the molecule in the surface adsorbed state. The genesis of selective enhancements of the Raman bands in the SERS spectra emanating from the cationic, neutral, ylidic and anionic forms of the molecule have been divulged from the view of the Albretcht’s “A” and Herzberg−Teller (HT) charge transfer (CT) contribution. mechanism is based on the increase in the local electric field around the metal-molecule asdsorbed system, because of the excitations of localized surface plasmons resonance (LSPR). The CHEM mechanism, also referred to as the charge transfer (CT) mechanism, involves the photoinduced transfer of an electron from the Fermi level of the metal to an unoccupied molecular orbital of the adsorbate or vice versa depending on the energy of the photon and electric potential of the interface.6 The imidazole is a five-member heterocyclic ring molecule, a component of amino acid histidine containing pyrrole and pyridine nitrogen atoms. The molecule plays many important roles in various biochemical processes7 and exhibits a prominent ligand for transition metal ions.8 It acts as a versatile binding site in different iron-heme, vitamin B12 and metalloprotein9 systems. The 1-methyl-2-mercaptoimidazole molecule has been reported to act as a potent antithyroid drug and it inhibits the rate of uptake of radioactive iodine by the human thyroid glands.10 Imidazole derivatives have a wide range of pharmacological activity and are reported to have vasodialating, analgesic, antiinflammatory,11 cardiovascular,12 antineoplastic,13 antifungal,13,14 enzyme inhibition,15 antianthelmintic,16 anticancer, antiviral, and antiulcer activity. Other than their pharmacological actions, they also function as dyestuffs catalysts and polymerizing agents. Moreover, imidazole, mercaptoimidazole, and their

1. INTRODUCTION Vibrational spectroscopy has become an invaluable tool for identifying the structure of the molecules.1 Raman, IR, and sometimes both the spectroscopic techniques have enabled the spectroscopists to elucidate the protonation effects as well as the tautomeric preference of complex organic and inorganic molecules over the past decades.2 Moreover, proper assignments of the vibrational signatures and estimation of accurate force field for a molecule are of fundamental importance in vibrational spectroscopy. Recently density functional theory (DFT) and ab initio calculations are extensively utilized for the computation of vibrational frequencies and elucidation of thermodyamic and structural details of molecules.3 Because the normal Raman scattering (NRS) is a very weak scattering process, the NRS spectra are often masked by fluorescence emission. The potential to mingle the sensitivity of fluorescence with the structural information content makes surface enhanced Raman scattering (SERS) spectroscopy a powerful tool in a variety of fields, including drugs and medicine.4 It is an ultrasensitive vibrational spectroscopic technique not only for studying the molecules or ions at trace concentrations down to single molecule detection level5 but also for estimating the molecular forms and for their possible orientations on the metal surface.2,3 However, the physics behind the huge enhancement in Raman cross sections found in SERS is still a matter of debate. To date, it is generally recognized that two enhancement mechanisms, one a long-range electromagnetic (EM) effect, and the other a short-range chemical (CHEM) effect, are simultaneously effective. The EM © 2012 American Chemical Society

Received: July 16, 2012 Revised: October 19, 2012 Published: October 23, 2012 10934

dx.doi.org/10.1021/jp307024t | J. Phys. Chem. A 2012, 116, 10934−10947

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Figure 1. Optimized molecular structure of the (a) cationic, (b) anionic, (c) normal, and (d) ylidic forms of the molecule obtained from B3LYP/aug-ccPVTZ level of theory.

2. EXPERIMENTAL SECTION 2.1. Materials. The 2-MI molecule was purchased from Aldrich Chemical Co. and was used without any further purification. Silver nanocolloids were prepared by the process of Creighton et al.20 The stable yellowish sol thus prepared showed a single extinction maximum at 392 nm and it was aged for 2 weeks before being used in the experiment. The size of the silver particles in this sol is known to be in the range 1−50 nm.21 All required solutions were prepared with distilled and deionized water from a Milli-Q-plus system of M/S Millipore Corp., USA. Mixing a specific volume of stock solution with an appropriate volume of silver nanocolloid attained the desired concentration of the probe molecule in silver colloid. 2.2. Instrumentation. Raman spectra were recorded by a Spex double monochromator (Model 1403) fitted with a holographic grating of 1800 grooves/mm and a cooled photomultiplier tube (Model R928/115) from Hamamatsu Photonics, Japan. The sample was taken in a quartz cell and was excited by 514.5 nm radiation from a Spectra Physics Ar+ ion laser (Model 2020-05) at a power of 200 mW. Raman scattering was collected at a right angle to the excitation. The operation of the photon counter and data acquisition and analysis were controlled by Spex Datamate 1B. The acquisition time by the spectral element was 0.5 s. The scattered light was focused onto the entrance slit of ∼4 cm−1 width. Polarized Raman spectra were recorded with an arrangement provided with the instrument. The accuracy in the measurement was ±1 cm−1 for strong and sharp bands and slightly less for other bands. The FTIR spectra of the powder samples were taken in a KBr pellet using a Nicolet Magna-IR 750 spectrometer series II. The resolution of the infrared band was about 4 cm−1 for sharp bands and slightly less for broader bands. The electronic absorption spectrum was recorded in a Shimadzu spectrophotometer Model UV−vis 2010

derivatives are very stable toward reducing or oxidizing agents and are commonly used as corrosion inhibitors as they adsorb strongly on a variety of metals.17 Considering the enormous biological and industrial importance, this paper is mainly focused to understand the preferential existence of the cationic, neutral, ylidic, and anionic forms of the 2-mercaptoimidazole (2-MI) molecule in solid and in aqueous solution. Experimental supports to comprehend the existence of the preferential forms of the molecule were gained from the FTIR and NRS spectra. To the best of our knowledge, except for the paper of Perez-Pena et al.18 no vibrational analysis of the molecule has been reported so far. The manuscript may be considered as the first detail report of the complete vibrational assignment of the molecule. Further insight into the molecular structures and the theoretical estimation of vibrational signatures of different forms of the molecule are provided from the density functional theory (DFT) calculations. The adsorptive behavior of the preferential form/forms of the molecule on the nanocolloidal silver surface at different concentrations of the adsorbate, close to that encountered under physiological conditions in living systems, have been elucidated from the SERS spectra. In these investigations, the nanocolloidal silver surface may emulate an artificial biological interface, and after elucidating the adsorption mechanism of the molecule, the study can be expanded to the adsorption on membranes or other interesting biological surfaces for medical or therapeutic treatments.19 The selective enhancements of the Raman bands in the SERS spectra of the molecule have been explored from the view of the Albretcht’s “A” and Herzberg−Teller (HT) CT contribution. 10935



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Figure 2. (a) Normal Raman spectra of the molecule in 0.1 M aqueous solution (λexc = 514.5 nm). The theoretically simulated gas-phase Raman spectrum of the (b) cationic, (c) normal, (d) ylidic, (e) anionic, and (f) mixed forms of the molecule calculated using DFT.

PC. All the spectra reported in the figures are original raw data directly transferred from the instrument and processed using the Microcal origin version 6.0.

software25 from the output of the DFT calculations. Cartesian displacement and calculated vibrational modes of the molecule have been displayed using Gauss View-03 software. Information pertaining to the excited states of the anionic form of the molecule was obtained by calculating the Franck−Condon transition energies for the B3LYP/aug-cc-PVTZ optimized ground state structures at the configuration interaction single (CIS)/aug-cc-PVTZ level of theory. In the process of geometry optimization for the fully relaxed method, convergence of all the calculations and the absence of imaginary values in the wavenumbers confirmed the attainment of local minima on the potential energy surface. The solvent effects on the structure of the molecule were estimated by the integral equation formalism polarized continuum model (IEFPCM), which is a special version of the polarized continuum model (PCM). Detail

3. COMPUTATIONAL DETAILS The theoretical calculations were carried out using Gaussian 03 operated in the Linux operating system.22 The relevant molecular structures were optimized and the normal modes of vibrations of the molecule at the preoptimized structures were estimated by DFT calculations. The B3LYP functional23 and augcc-PVTZ24 basis set were used in the DFT calculations. The theoretically estimated vibrational frequencies for all the probable forms of the molecule were presented without using any scaling factor. The PED calculations in terms of internal coordinates of the molecule were performed with GAR2PED 10936


10937

a

1486 vs 1548 vw

1368 w 1452 w 1486 vvs

1121 s 1231 vs 1326 w

1116 ms 1229 vvs

1340 ms

A′ A′/A″

952 ms 974 w

951 w

A′ A′ A′ A′ A′ A′

A′ A′/ A1 A′/ A1

A′/A″ A′/A″/ B1

A′ A′/ A″/B1 A′/ A″ A″/ A2 A′/A″

180 vw 230 vw 509 w 610 w 663 vw

735 vw

235 w 517 ms 619 ms

NRS solid

symmetry species (Cs/ C2v)

NRS Solution

1548 2659

1365 1452

1121 1223

975

750

688

217 481

calc Raman 2-MI

64 str (CN/CC) 100 str (SH)

78 str (CN) 71 str (CN)

84 (NCH i.p. bend) 74 (CNH i.p. bend)

72 (C−N−C i.p. bend)

71 [tor (HCNH); torring] 91 tor (HCCH)

84 torring 83 str (CS)

assignment (PED)%

1545

1349

1240

991

757 784

189 209 478 633 673

calc Raman 2MI yide

84 (CNH i.p. bend)

64 (NCH i.p. bend)

66 (CNH i.p. bend)

76 tor (HNCH)

79 (CNH i.p. bend) 73 (CC−H/CNH i.p. bend)

62 (NCN i.p. bend) 92 (CSH i.p. bend) 81 (SCN i.p. bend) 92 (NH o.p. bend) 89 (CN−H i.p. bend)

assignment (PED)%

2652

1438 1482

1321

1114

958 986

733 775

218 483 626 669

calc Raman 2-MI+

96 str (SH)

95 (CNH i.p. bend) 64 str (CC/CN)

59 (CCH/NCH i.p. bend)

88 (CSH i.p. bend) 84 (CCH/CNH i.p. bend) 87 (CNH i.p. bend)

84 (NH o.p. bend) 86 tor (HCCH)

70 torring 80 str (CS) 93 tor (HCCH) 79 (NH o.p. bend)

assignment (PED)%

1229 1314

793

620

221

calc Raman 2-MI‑

92 str (CS) 66 (CCN i.p. bend)

89 (CH o.p. bend)

83 tor (HCCN)

64 tor (HCCH)

assignment (PED)%

Key: vs, very strong; s, strong; ms, medium strong; w, weak; vw, very weak; vvw, very very weak; str, stretching; i.p. bend, in-plane bending; o.p. bend, out-of-plane bending; tor, torsion.

2653 w

1485 s

1232 s

733 ms 791 w

673 w

51 5w

FTIR (obs)

Table 1. Observed and Calculated IR and Raman Bands of the Molecule in Varied Environment and Their Tentative Assignmentsa

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of Figure 2. The theoretically simulated NRS spectra of the cationic, normal, ylidic, anionic, and mixed forms of the molecule in the gas phase are shown in panels b−f of Figure 2, respectively. The underlying aim of recording the FTIR and NRS spectra is to apprehend the existence of the preferential form/forms of the molecule in solid state and in aqueous solution from the assignment of the vibrational signatures. Table 1 lists the experimentally observed selected FTIR and NRS band frequencies and the theoretically simulated vibrational frequencies of the cationic, normal, ylidic and the anionic forms of the molecule in the gas phase. Table S1 (Supporting Information) lists the detail vibrational analysis for the same along with their tentative assignments of the molecule as provided from the potential energy distributions (PED). The observed disagreement between the theory and the experiment could be a consequence of the anharmonicity. Moreover, the quantum chemical methods have a general tendency of to overestimate the force constants at the exact equilibrium geometry.29 However, it is to be emphasized that the calculated Raman spectrum represents the vibrational signatures of molecules in its gas phase. Hence, the experimentally observed NRS of solid and solution may differ significantly from the calculated spectrum. Despite that, one can see that there is a general concordance regarding the Raman intensities as well as the position of the peaks between the experimental and calculated spectra.30 Compared to the NRS spectrum of the molecule recorded in solid state (Figure S1(b), Supporting Information), the IR spectra of the powdered sample (Figure S1(a), Supporting Information) are characterized by sharp and well resolved vibrational bands. The modes arising principally from the torsion, stretching, and bending vibrations of the imidazole ring moiety and the externally attached −SH group of the molecule are identified. In assigning the vibrational frequencies, literature concerning the normal coordinate analysis and vibrational assignments of the imidazole and 2-mercaptoimidazole molecules has been considered.18,29,30 The bands at ∼673 cm−1 (calculated at 688/673/669 cm−1 for 2-MI/yilide/2-MI+ forms of the molecule) and ∼733 cm−1 (calculated at 750/757/733 cm−1 for 2-MI/ylide/2-MI− forms of the molecule) exhibit medium and strong transmittance in their IR spectra. respectively. These bands are hardly observed in the NRS spectrum of the molecule recorded in the solid state and are ascribed to a prevailing contribution from the in-plane bending vibration of the ylidic form and the out-of-plane (N−H) bending, torsion of the imidazole ring moiety emanating from the 2-MI and 2-MI+ forms of the molecule, respectively. The assignment of 733 cm−1 band is at variance with that reported by Perez-Pena et al., who ascribed this band to the out-of-plane C−H bending mode.18 A weak, but distinct, band at ∼791 cm−1 (calculated at 775/784/793 cm−1 for 2-MI+/ylide/2-MI− forms of the molecule) is recorded in the IR spectrum of the molecule. This may be considered to originate either from the ylidic or from cationic, anionic forms of the molecule. As in the solid state, the presence of cationic and/or anionic forms of the molecule may be excluded, thereby signifying its origin from the ylidic form of the molecule. Considerable attention can be drawn for the band centered at ∼1232 cm−1 (calculated at 1223/1240/1229 cm−1 for 2-MI/ylide/2-MI− forms of the molecule), which appears as the strong signal both in the IR and in the NRS spectrum of the molecule. This band has been attributed to a dominant contribution from the in-plane C−H/α(C1−N9−H10)/α(N4− C1−N8) bending vibrations emanating from the 2-MI/ylidic/2-

discussions of the IEFPCM model have been reported elsewhere.26 The multivariate data analysis has been employed using the Unscrambler software version 9.7.27 The pKa value of the molecule was theoretically simulated using the Marvin 5.1.0 software.28

4. RESULTS AND DISCUSSION 4.1. Molecular Structure. The 2-MI consists of a planar nonaromatic penta-atomic imidazole ring containing two nitrogen atoms (ca. N5 and N2) along with an externally attached mercapto (ca. −SH) group. In the heterocyclic imidazole ring, the N5 and N2 nitrogen atoms are designated as the pyrrole and the pyridine nitrogen respectively. The N5−H and S6−H ionizable functional groups of the molecule have pKa1 and pKa2 values at ∼5.58 and 7.80, respectively, as obtained from the theoretical simulations.1 Thus, depending upon the pH of the medium, the molecule can exist in cationic (2-MI+), normal (2MI), and anionic (2-MI−) forms. However, as in imidazole,7−10 the normal form of the molecule can also coexist in prototropic tautomeric equilibrium with its corresponding ylide form. The optimized molecular structures of the 2-MI+ and the 2-MI− forms are shown in Figure 1, panels a and b, respectively. The 2-MI and its corresponding tautomeric ylide forms are also shown in panels c and d of Figure 1. To retrieve some ideas concerning the relative stability of different forms of the molecule in the gas phase, minimum energies of the molecule at their respective optimized geometries have been computed using the DFT method. The normal, ylidic, and anionic forms of the molecule are calculated to have more SCF energies than the most stable cationic form of the molecule with ΔE = 10.13, 12.05, and 42 eV, respectively. 4.2. Normal Raman and FTIR Spectra of the Molecule and Their Vibrational Assignments. The normal (2-MI)/ ylidic and the cationic (2-MI+) forms of the molecule approximately belong to Cs point group symmetry, and have 10 and 11 atoms, respectively. Thus the (2-MI)/yilidic, the cationic (2-MI+) forms have 24 and 27 fundamental vibrations that are distributed among the symmetry species as 17 A′ + 7 A″ and 19 A′ + 8 A″, respectively. However, the anionic (2-MI−) form of the molecule has 8 atoms and belongs to C2v point group symmetry with C2 points toward the Cartesian z axis, the yz plane being the imidazole ring plane of the molecule. The 18 normal modes of vibrations for this preferred orientation of the molecule are classified as Γvib = 7A1 + 3B1 + 2A 2 + 6B2

Among these vibrations, the modes represented by the totally symmetric A1 symmetry species are expected to be polarized in the Raman spectra. The cationic, normal/ylidic, and anionic forms of the molecule do not have any center of inversion; hence, all the fundamental vibrations emanating from these molecular structures are expected to appear both in the Raman and in the FTIR spectra. However, among these vibrations some normal mode/modes may emanate either from the cationic, normal/ ylidic, and anionic forms or from two, three, or all four possible forms of the molecule. Thus, the appearance of some degenerate normal modes in the vibrational signature of the molecule cannot be precluded. The FTIR spectra of powdered molecule in KBr pellet and the corresponding NRS spectrum of the molecule in neat solid are shown in panels a and b of Figure S1, respectively, in the Supporting Information. The NRS spectra of the molecule at 0.1 M aqueous solution in neutral pH medium are shown in panel a 10938



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Figure 3. SERS spectra of the molecule at varied adsorbate concentrations (λexc = 514.5 nm).

MI− forms of the molecule. The assignment of this band is in partial agreement with that reported by Perez-Pena et al.18 who ascribed this band to in-plane D (CH) deformation vibrations.

Interesting conclusion can be drawn from the appearance of a weak but distinct band at ∼2650 cm−1 (calculated at 2659/2652 cm−1 for 2-MI/2-MI+ forms of the molecule) in the IR spectrum 10939



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concomitant presence of the normal, ylidic, and/or cationic forms of the molecule at the neutral pH (ca. pH∼7) of the medium. The vibrational analysis of the experimentally observed NRS spectra of the molecule aided by the DFT calculations thus allows us to identify the marker bands whose vibrational signatures stem from the cationic, normal, ylidic, anionic, and/or all the probable combinations of the above-mentioned forms of the molecules. However, the respective population of the various forms of the molecule may vary in the aqueous solution. The quantitative measure of relative population of different forms of the molecule in aqueous solution can be estimated from the ratio of the sum of the integrated intensities of the assigned experimental bands representing vibrational signatures of the − normal (ca. 2-MI) (I2−MI), anionic (ca. 2-MI−) (I2‑MI ), ylidic

of the molecule recorded in the solid state. The band has been ascribed to a prevailing contribution from the ν(S−H) stretching vibration and marks the presence of the normal and/or cationic forms where the hydrogen atom in the mercapto group of the molecule remains undissociated. Perez-Pena et al. attributed the ν(S−H) stretching vibration to a band that appears as a weak signal in the IR spectra of the molecule recorded at ∼2532.6 cm−1 in the solid state.18 A weak band at ∼3633 cm−1 (calculated at 3648/3617 cm−1 for 2-MI/ylide forms of the molecule) has been ascribed to a predominant contribution from the ν(N−H) stretching vibrations emanating from the normal and/or ylidic forms of the molecule. Interestingly, the presence of the weak band may indicate the possibility of H-bonded dimers between N−H on one normal/ylidic form and the unprotonated N atom of the other normal form of the molecule. Thus, the analyses of the NRS and IR spectra primarily presage the presence of normal, ylidic, and/or cationic form of the molecule in the solid state. The presence of the cationic form may be precluded in the powder form of the molecule, thereby signifying the prevalence of either the normal (ca. 2-MI) or ylidic forms of the molecule in the solid state. The spectral features of the NRS spectrum of the molecule at 0.1 M in aqueous solution at neutral pH (Figure 2a) in general are poor, albeit characterized by some intense vibrational signatures. Medium intense and weak Raman bands are recorded at ∼952 cm−1 (calculated at 958 cm−1 for the 2-MI+ form of the molecule) and at ∼509 (calculated at 481/478/483 cm−1 for 2MI/2-MI+ forms of the molecule). The former band has been attributed to a significant contribution from the in-plane α(C1− S5−H9) bending vibration originating from the cationic form of the molecule whereas the later has the prevailing contribution from the ν(C1−S6) stretching/in-plane α(N4−C1−S5) bending/ ν(C1−S5) stretching vibrations emanating from the normal/ ylidic/cationic forms of the molecule. However, Perez-Pena et al.18 assigned the IR and Raman bands of the molecule recorded at ∼955 and 953.1 cm−1 as the γ(N−H) vibration, which is not in agreement with our PED calculations. The presence of the cationic form of the molecule is corroborated by the appearance of very strong Raman band at ∼1486 cm−1 (calculated at 1482 cm−1 for the 2-MI+ form of the molecule). This band is ascribed to a predominant contribution from the ν(C−C)/ν (C−N) stretching vibration emanating from the cationic form of the molecule. The other prominent Raman band of the molecule in aqueous solution at neutral pH is observed at ∼1231 cm−1 (calculated at 1223/1240/1229 cm−1 for 2-MI/ylidic/2-MI− forms of the molecule). At the neutral pH of the medium, the presence of the anionic form is unfeasible whereas the existence of the normal and/or ylidic form of the molecule is expected to be preponderant. Hence, the vibrational signature of 1231 cm−1 may be considered to arise either from the in-plane C−H and stretching ν(C 1 −N 5 ) or from α(C 1 −N 9 −H 10 ) bending vibrations of the normal and the ylidic forms of the molecule, respectively. The existence of the normal, ylidic, and/or cationic forms of the molecule is, however, substantiated by the presence of weak Raman bands at ∼974 cm−1 (calculated at 975/991/986 cm−1 for the2-MI/ylidic/2-MI+ forms of the molecule) and 1121 cm−1 (calculated at 1121/1114 cm−1 for the 2-MI/2-MI+ forms of the molecule), whereas the exclusive presence of the normal form of the molecule is justified by the appearance of 1368 cm−1 (calculated at 1365 cm−1 for the 2-MI form of the molecule) and 1452 cm−1 (calculated at 1452 cm−1 for 2-MI form of the molecule) bands. The above vibrational analyses thus divulge the

+

(Iylidic), and cationic (I2‑MI ) forms of the molecule divided by the theoretically predicted sums of their respective absolute − intensities (ca. A2‑MI , A2‑MI−, Aylidic, A2‑MI+) taking two distinct forms of the molecule at a time: ∑i Ii ∑ Aj [Formi] = ∑j Ij ∑ Ai [Formj]

(1)

Figure S2 (Supporting Information) shows the bar diagram indicating the relative population of the cationic, normal, ylidic, and anionic forms of the molecule in aqueous solution at neutral pH. The results indicate that 43% of the normal, 25% of the cationic, 21% of the ylidic, and 11% the anionic forms of the molecule are prevalent in aqueous solution at neutral pH. This result is in accordance with our earlier conjecture as predicted from the vibrational assignment (vide supra). 4.3. Concentration Dependent SERS Spectra of the Molecule. The concentration dependent SERS spectra of the molecule as shown in Figure 3 are characterized by a number of sharp and well resolved Raman bands particularly in the wavenumber range 1000−1700 cm−1 having increased S/N ratio compared to the NRS spectrum. The relative intensities of some of these bands vary with the concentrations of the adsorbate. Considerable attention can be drawn regarding the enhancement of SERS band at ∼1360 cm−1. This band has been assigned to a prevailing contribution from the ν(C−C/C−N) stretching vibration and marks the presence of the normal form of the molecule in the surface adsorbed state. The presence of the normal and/or ylidic form of the molecule is epitomized by the significant enhancement of band at ∼1575 cm−1. This band may be considerably blue-shifted with respect to its NRS counterpart, which appears as a weak signal at ∼1547 cm−1 in solid state. This band has been assigned to a predominant contribution from the ν(C−N) stretching and in-plane bending vibration involving the nitrogen atom emanating from either the normal or ylidic form of the molecule respectively. The significant enhancement of the above-mentioned Raman bands not only signifies the presence of normal and/or ylidic form of the molecule in the surface adsorbed state but also entails considerable involvement of the nitrogen atom of the molecule in the adsorption process. The involvement of nitrogen atom in the adsorption process may result in the appearance of shoulder at ∼21631 cm−1 assigned to the ν(Ag−N) stretching vibration. A significant conclusion can be drawn regarding the enhancement of the SER bands at ∼610 and 770 cm−1. The corresponding NRS counterpart of the enhanced 610 cm−1 SER band appears as a very weak signal, hardly recovered from the noise, at ∼619 and 610 cm−1 10940



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(calculated at 633/626/620 cm−1 for ylidic/2-MI+/2-MI− forms of the molecule) in the solid state and in aqueous neutral solution, respectively. The band has been ascribed to the in-plane N−H bending vibration originating from the ylidic form of the molecule and/or torsional motion stemming from the cationic/ anionic forms of the molecule. The very weak intensity of the band in the NRS spectra of the molecule steers us to believe its origin from the out-of-plane vibration of the cationic and/or anionic form of the molecule. The other band at ∼770 cm−1 in the SERS spectra of the molecule, recorded at various concentrations of the adsorbate, is considerably broadened and has no distinct NRS counterpart in the aqueous solution. Considering the pH of the medium, this band has been assigned to have prevailing contribution from the in-plane bending and/or torsional, out-of-plane C−H bending vibrations, originating either from the ylidic or from the cationic and anionic forms of the molecule. However, the broadening of the vibrational signature in the SERS spectra may indicate considerable interaction of the imidazole ring moiety with nanocolloidal silver surface. This observation may portend that this normal mode may emanate from the out-of-plane torsional vibration of the imidazole ring moiety of either the cationic or the anionic or both forms of the molecule. The existence of the anionic form of the molecule in the surface adsorbed state can be further envisaged from significant enhancement of 1309 cm−1 band (calculated 1314 cm−1 for 2-MI− forms of the molecule) in the entire concentration dependent SER spectral profile. This band has been assigned to the α(N−C−C) bending vibration emanating from the anionic form of the molecule and its enhancement further signify the involvement of the lone pair electrons of the nitrogen atom in the adsorption process. The exclusive presence of the cationic form of the molecule in the entire concentration dependent SER spectral profile may be envisaged from the significant enhancement of the Raman band at ∼1507 cm−1. This band has been considerably blue-shifted with respect to its NRS counterpart in solution, which appears as a strong signal at ∼1486 cm−1. The other striking feature in the entire concentration dependent SERS spectra of the molecule is the appearance of a sharp and significantly enhanced band at ∼1649 cm−1. There is an ambiguity in the assignment of this band. This band has been reported to originate from the normal and cationic as well as from the anionic forms of the imidazole molecule,32 albeit the theoretical calculations do not estimate the normal mode in this frequency region. Considering the pH of the medium, the enhancement of the above band may connote the adsorption of the cationic, normal and the anionic forms of the molecule in the adsorption process. The above analyses of the concentration dependent SERS spectra of the molecule connote the concomitance of the cationic, normal, ylidic, and anionic forms of the molecule in the surface adsorbed state. The relative intensity patterns of the above-mentioned SERS bands changes with the concentration of the adsorbate. However, to elucidate the preferential existences of particular form/forms of the molecule with adsorbate concentration, the multivariate data analysis technique has been applied. In the multivariate analyses, the X-matrix (data matrix) consist of 7 objects (n = 7) each of which designates vibrational mode emanating from the cationic, anionic, normal, ylidic, or either of the two forms of the molecule. For example, the acronyms for the normal modes signifying the vibrational signatures originating from the cationic, normal, anionic, normal and/or ylidic, cationic and/or anionic, and anionic and/or

normal forms of the molecule are represented as CA, N, A, N/Y, CA/A, and A/N, respectively. In the predefined X-matrix, 8 variables (p = 8) designated as C1 to C8 represent the SERS intensity of each object at 1.0 × 10−8, 1.0 × 10−9, 1.0 × 10−10, ..., 1.0 × 10−15 M concentrations of the adsorbate, respectively. The PC1 vs PC2 plot representing the scores and loadings of the defined objects and variables on the t1, t2 plane is shown in Figure 4. Here PC1 is the most dominant principal component and it

Figure 4. PC1 vs PC2 plot representing the scores and loadings of the defined objects and variables on the t1, t2 plane.

carries 98% of the total X-variance. From Figure 4, it is seen that acronyms CA, N, A, N/Y, CA/A, and A/N are relatively disposed in the t1, t2 plane along with the variables ranging from C1 to C8. This result further signifies the presence of all the possible forms of the 2-mercaptoimidazole molecule in the surface adsorbed state at various concentrations of the adsorbate. In this connection, it may be mentioned that the concomitant presence of the various forms in the case of the parent imidazole molecule is reported elsewhere.33 4.4. Orientation of the Molecule on the nanoColloidal Silver Surface. It is enigmatic to elucidate the precise orientation of the predicted forms of the molecule on the nanocolloidal silver surface. However, to have an idea concerning the orientation of the different forms of the molecule at varied concentrations of the adsorbate, we estimate the apparent enhancement factors (AEF) of some selected Raman bands using the relation we reported elsewhere.12 Accordingly, AEF = σSERS[C NRS]/σNRS[CSERS]

(2)

where C and σ represent the concentration and the peak area of the Raman bands measured from baseline. They are shown in Table 2. The orientation of the molecule has been estimated following the surface selection rule, as predicted by Moskovits.34 According to this rule, the normal modes of vibrations with the polarizability derivative components perpendicular to the surface will be enhanced more. If the imidazole ring plane of the cationic, normal, and ylidic forms of the molecule (all approximately belonging to Cs point group symmetry) is considered to be lying in the xy plane and z is perpendicular to the molecular plane, then for the edge-on adsorption, the vibrations of the in-plane A′ species spanning as xx or yy (depending upon the stance of the molecule on the colloidal silver surface) are expected to undergo significant enhancement. For the face-on adsorption stance of the molecule on the nanocolloidal surface, the vibrations of the out-of-plane A″ species transforming as yz and xz are expected to be enhanced. The anionic form of the molecule, however, belongs 10941



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Table 2. Apparent Enhancement Factors (AEF) and Probable Tensor Elements (PTE) of Some Selected Raman Bands of the Molecule at Varied Adsorbate Concentrations adsorbate concentrations NRS solution (cm−1)

SERS (cm−1)

610

610

777

771

1326 1368 1486 1590

1310 1361 1508 1573

PTE (sym) (C2v / CS) (αxz, αyz; αzx) (A2, A″) (αyz, αyz; αzx) (B1, A″) αxx; αyy (A1, A′) αxx; αyy (−, A′) αxx; αyy (A1, A′) αxx; αyy (−, A′)

10−8M

10−10M

10−11M

10−12M

10−13M

10−14M

5.6 × 10

7.6 × 10

7.5 × 10

4.1 × 10

3.2 × 107

4.0 × 108

4.3 × 109

4.7 × 1010

3.1 × 107 7.3 × 107 1.6 × 107 3.7 × 107

3.3 × 108 7.1 × 108 1.7 × 108 3.5 × 108

5.8 × 109 1.4 × 1010 3.4 × 109 7.2 × 109

5.2 × 1010 1.3 × 1011 2.9 × 1010 6.3 × 1010

5.6 × 10

10−9M 6

7

8

to C2v point group symmetry, and it is considered to be lying in the yz plane, x is perpendicular to the molecular plane, and z corresponds to the C2 axis. For the face on adsorption of the anionic form of the molecule, the vibrational modes of the irreducible representations A2 and B1, transforming as xz and yz, respectively, are likely to be enhanced. From Table 2, it is clearly seen that we obtain a significant 7− 14 orders of magnitude enhancement of the SERS bands centered at ∼1360 and 1575 cm−1 principally representing the inplane vibrations (A′) emanating from the normal and/or ylidic forms of the molecule, respectively. Similar enhancement factors are also estimated for the in-plane vibrations at ∼1309 and 1507 cm−1 originating from the anionic and cationic forms (A1, A′) of the molecule. Moreover, the out-of-plane modes at ∼610 (A2, A″) and 771 cm−1 (B1, A″) emanating from the anionic and/or cationic forms of the molecule, respectively, are also significantly enhanced. The significant enhancement of the in-plane bending vibrations centered at ∼1360 and 1575 cm−1, together with the appearance of Ag−N stretching at ∼216 cm−1 31 vibrations may signify the edge-on adsorption of the normal and/or ylidic form of the molecule through its lone pair electrons from the N (pyrrole/pyridine) atom. However, for the anionic and/or cationic forms of the molecule, significant enhancement of both the in-plane and out-of-plane modes (Table 2) presage tilted or nearly flat orientation of the molecule with respect to the nanocolloidal silver surface. The tilted or nearly flat orientation of the imidazole molecule on copper and gold electrodes and on silver colloid were reported elsewhere.35 The face-on adsorption stance of the anionic and/or cationic forms of the molecule may result in the involvement of sulfur atom in the adsorption process in addition to the nitrogen atom. The involvement of sulfur atom in the adsorption process can be envisaged from the appearance of kinks and flexes in the frequency region ∼180−200 cm−1 ascribed to Ag−S stretching vibration36 in the SERS spectra of the molecule recorded at various concentrations of the adsorbate. The downward shifts of SER bands involving C−S vibrations are however not observed. This may be due to the overlapping of the enhanced vibrational signatures emanating from the various forms of the 2-mercaptoimidazole molecule in the surface adsorbed state and the bands involving C−S vibrations could not be identified unambiguously. The relative variation in intensities and enhancement factors of the SERS bands at various adsorbate concentrations (ca. from Metal to Molecule) may indicate fluxional motion of the various forms of the molecule (ca. normal, ylidic, cationic and anionic forms) on the nanocolloidal silver surface. 4.5. CT Contribution to the SERS of the Molecule. To elucidate the selective enhancement of Raman bands in the SERS

9

10−15M

4.5 × 10

12

5.2 × 10

2.9 × 1013

2.7 × 1011

3.1 × 1012

3.7 × 1013

2.3 × 1014

2.3 × 1011 6.4 × 1011 1.5 × 1011 3.2 × 1011

3.8 × 1012 9.8 × 1012 2.1 × 1012 5.7 × 1012

3.4 × 1013 8.1 × 1013 1.6 × 1013 3.5 × 1013

2.7 × 1014 7.2 × 1014 1.5 × 1014 3.5 × 1014

10

11

spectra of the molecule, the SERS-CT mechanism as proposed by Lombardi et al.37 has been considered. The CT model of SERS based on the resonance Raman (RR) theory of Albrecht has been proposed.38 As in RR, the Raman polarizability tensor elements (ασρ) in SERS are also represented by the Albrecht’s “A”, “B”, and “C” terms. Term “A” represents the Franck− Condon (FC) contribution and only totally symmetric vibrational modes are expected to be enhanced by this mechanism as in the case of RR. The terms “B” and “C” in SERS, arise from the Herzberg−Teller (HT) contribution. The HT contributions are considered to be responsible for the selective enhancement of the Raman bands via molecule to metal CT and metal to molecule CT, respectively.39 Both totally and nontotally symmetric vibrational signatures may be enhanced via “B” and “C” terms.38,39 The face-on adsorption of the anionic and the cationic forms of the molecule (vide ante) may signify considerable interaction of the π electron cloud of the imidazole ring moiety of the molecule with the nanocolloidal silver surface. Moreover, the appearance of the ν(Ag−N) and ν(Ag−S) vibrations at ∼216 and 180 cm−1 37−39 in the entire concentration dependent SERS spectra of the molecule also connotes considerable involvement of the CT mechanism to the overall SERS enhancement of the molecule. The direction of CT interaction for the various forms of the 2-mercaptoimidazole may be considered to be from metal to molecule like that for imidazole and cyanoimidazole molecules reported elsewhere.40 Moreover, the imidazole molecule acts as a π-acceptor ligand and the corresponding MLCT states are extensively reported.41 Term “A” in the case of CT from a filled Fermi level |F⟩ of the metal nanocolloid to the excited molecular state |K⟩ is represented by37,39 σ ρ A = (2/ℏ)μFK μFK ⟨i|k⟩⟨k|f⟩

ωFK + ωf (ωFK + ωf )2 − ω 2

(3)

where the symbols have the same meaning as reported elsewhere.37,39 The eq 3 representing the Albrecht’s “A” term for SERS will be finite if the dipole transition moments μσFK and μρFK and the vibrational overlap integral (FC factors) ⟨i|k⟩⟨k|f⟩ are concurrently nonzero. For the FC factors to become nonzero, at least one of the following two conditions is to be fulfilled: (i) There must be a displacement of the minima in the potential energy surfaces (PESs) of the two states involved in the transition (ΔQK ≠ 0) along a given normal coordinate Q. (ii) There must be a change in the curvature of the PES (ΔνK ≠ 0). 10942



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Figure 5. Cartesian displacement and calculated (B3LYP/aug-cc-PVTZ) vibrational modes of (a) 2-MI− and (b) 2-MI+ molecules. The numbers in the parentheses referred to the experimental values of the assigned band (upper panel). Shape of the LUMO orbital of (c) 2-MI− and (d) 2-MI+ molecule (lower panel).

where K is a constant and νi is the vibrational frequency of the ith normal mode in the S0 state. The ΔQ values corresponding to the D0−S0 transition of the molecule, calculated for the totally symmetric normal modes A′ emanating from the cationic/normal, normal, cationic, and normal/ylidic forms of the molecule centered at ∼1124, 1361, 1508, and 1573 cm−1, are −1.85/0.041 (cationic/normal), −0.001, 0.918, and 0.0106/0.021 (normal/ylidic) amu1/2 Ǻ , respectively. Apart from these, the ΔQ value for the totally symmetric vibrational signature at ∼1310 cm−1 (A1) representing the anionic form of the molecule are estimated to be −0.203 amu1/2 Ǻ . The appreciable ΔQ values for the bands centered at ∼1508 and 1310 cm−1 originating from cationic and the anionic form of the molecule, respectively, indicate that the enhancement of these bands in the SERS spectra of the molecule may primarily be due to the contribution from the Albrecht’s “A” term. In the SERS-CT mechanisms, the selective enhancement of the above-mentioned SER bands (ca. at 1508 and 1310 cm−1) may indicate the presence of the forces acting on the molecule in the excited state. These forces depend on the difference between the equilibrium geometries of the adsorbate (may be cationic, normal and anionic−) and their respective anions, which are then explored through the ΔQ vector.43 They are closely related to the shape of the virtual orbital (ca. the LUMO) of the adsorbate, shown in the Figure 5, where the transferred electron was in transit. When an electron is transferred from the cationic, normal, and anionic forms of the molecule to form their respective radical anion in a SERS-CT process, the nuclei are displaced from their equilibrium positions depending on the shape of the LUMO. From Figure 5, it is clearly seen that the LUMO is mainly located on the imidazole ring moiety of the cationic form whereas it is located around the H6−C2 and H7−C3 bonds in the anionic

The CT mechanism of SERS may be considered to be analogous to RR process, however, the difference is that in SERSCT, the transient exited state is a CT level of the metal− adsorbate (M−A) complex. There is a photoinduced transfer of an electron from the metal to the vacant orbital of the adsorbate yielding metal-adsorbate CT state. When the electron returns back to the metal, the electron−hole recombination creates a Raman photon but the molecule remains in the vibrationally excited state. Thus from the exclusive point of view of the adsorbed molecule, the resonant process takes place between the ground state of the various possible cationic, normal, ylidic, and anionic forms of the molecule (singlet S0 electronic ground state of the adsorbate) and their corresponding neutral, anion, anion, and dianion (doublet D0 electronic ground state of the adsorbate), respectively. The detailed analyses of the abovementioned model of the CT-SERS mechanism have been reported elsewhere.42 The displacement between the PES minima of the S0 and the D0 states can be expressed as a function of normal coordinates (Q) of the ground state by the relation

ΔQ = L−1ΔR

(4)

where L−1 is the inverse of the normal mode L matrix of the S0 state as obtained from the DFT calculations and ΔR is a vector that contains the differences between the optimized structures of the states involved in the D0−S0 transitions expressed in the same set of internal coordinates. The ΔQ values are related to the Raman activity of each fundamental under resonance conditions through the following equation:

Ii = K ΔQ i2νi3

(5) 10943



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form of the molecule. The spatial locations of the LUMO orbitals may result in the shortening or enlargement of the bond lengths between the atoms depending upon bonding or antibonding interactions of the respective π-orbitals. The resulting geometrical distortions of the molecule closely resemble the Cartesian displacements of atoms associated with the vibrational modes at 1508 and 1310 cm−1, as shown in Figure 5. These results thus connote the predominant contribution of Albrecht’s “A” term in the SERS-CT enhancement of the above-mentioned Raman bands. However, apart from the enhancement of the abovementioned vibrational signatures (ca. at ∼1508 and 1310 cm−1), the out-of-plane nontotally symmetric normal modes at ∼610 (A2, A″) and 771 cm−1 (B1, A″) emanating from the anionic and/or cationic forms of the molecule are also significantly enhanced. The enhancement of the nontotally symmetric modes in the SERS spectra may not be accounted for by the Albrecht’s “A” term contribution and primarily connote the CT effect of SERS through intensity borrowing from some allowed molecular transitions (the HT contribution) of the molecule.39,40 The direction of CT in the case of the 2mercaptoimidazole molecule adsorbed on silver nanocolloid is from metal to molecule (vide supra), hence the HT contribution stems from the Albrecht’s “C” term. Albrecht’s “C” term, according to the model of Lombardi et al,39,40 is represented as

Figure 6. Room temperature UV−Vis absorption spectra of the molecule at 1.0 × 10−5 M concentration in (a) acetonitile, (b) ethanol, and (c) water solvent.

the absorption maximum at ∼250 nm is of π → π* type. Table 3 shows the experimentally observed and theoretically predicted relevant allowed transitions along with the oscillator strengths and orbital symmetries between the low lying electronic states of the molecule. The HT selection rules may now be invoked (vide supra; eq 7). For the edge-on adsorption of the normal and the ylidic forms of the probe molecule on the nanocolloidal silver surface, the irreducible representation of the CT dipole moment operator Γ (μ⊥CT) = A′, whereas for the face on adsorption of the cationic and the anionic forms of the molecule, Γ(μ⊥CT) = A″ and A2, B1 respectively. Thus the CT states, according to HT selection rules must be of A′ (Γ(QK) = A′ ⊗ A′ = A′) symmetry corresponding to the transitions at 250 nm (calculated at 233 nm, f = 0.11, 1A′ ← 1A′ for the normal form; at 244 nm, f = 0.051, 1 A′ ← 1A′ for the ylidic form) and at 355 nm (calculated at 358 nm, f = 0.011, 1A′ ← 1A′ for the normal form) emanating from the normal/ylidic and normal forms of the molecule, respectively. The HT intensity borrowing from strongly allowed 1 A′ ← 1A′ transition, in turn allows the normal modes of vibration of the totally symmetric A′ species originating from the normal and the ylidic forms of the molecule to be enhanced significantly via the contribution from the Albrecht’s C term. Thus the enhancement of a totally symmetric 1573 cm−1 band in the SERS spectra of the molecule emanating from the normal and/or ylidic form of the molecule may be due to the HT (ca. Albrecht’s “C” term) CT contribution in addition to the EM mechanism. The involvement of the HT contribution toward its enhancement is further substantiated by the insignificant ΔQ values (vide ante), which thereby preclude the considerable contribution from the Albrecht’s “A” term. Interestingly, the CT states corresponding to the transitions at 250 nm (calculated at 232 nm, f = 0.9358, 1A′ ← 1A′ for the cationic form; calculated at 240 nm, f = 0.068, 1B2 ← 1A1 for the anionic form) and at 355 nm (calculated at 348 nm, f = 0.019, 1B2 ← 1A1 for the anionic form) emanating from the cationic and anionic forms of the molecule are A″ (Γ(QK) = A′ ⊗ A″ = A″) and B1, A2, (Γ(QK) = A2 ⊗ B2 = B1, Γ(QK) = B1 ⊗ B2 = A2), respectively. The HT intensity borrowed from strongly allowed 1 A′ ← 1A′, 1B2 ← 1A1 transitions, in turn allow the normal modes of vibrations of the A″ and A2, B1 symmetry species originating from the cationic and the anionic forms of the molecule, respectively, to be enhanced significantly via the contribution from the Albrecht’s C term. Thus the enhancement of the SER

⎛2⎞ C = −⎜ 2 ⎟ ⎝ℏ ⎠

∑

ρ σ [μKIσ μFK ](ωKIωFK + ω 2)hIF⟨i|Q |f⟩ + μKIρ μFK

(ωKI 2 − ω 2)(ωFK 2 − ω 2)

K≠I

(6)

where the symbols have usual significances as reported elsewhere.39 For the “C” term to be nonvanishing, the terms ⟨i|Q|f⟩, hIF, μKI, and μFK must be simultaneously nonzero, and this fundamental requirement leads to the HT surface selection rule. The HT surface selection rule is expressed as37,44 Γ(Q K ) =

⊥ )x ΓK ∑ Γ(μCT K

(7)

where Γ(QK) is the irreducible representation to which the SERS ⊥ active normal mode belongs. Γ(μCT ) is the irreducible representation to which the component of the CT dipole moment perpendicular to the surface belongs in the combined molecule-metal system, and ΓK is the irreducible representation of the molecular excited state to which the optical transition |I⟩ → |K⟩ is allowed. Figure 6 shows the absorption spectrum of the probe molecule recorded in various solvent media (ca. ethanol, acetonitrile, water) at neutral pH. The spectrum in aqueous medium, as shown in Figure 6 is characterized by intense and broad maximum at ∼250 nm along with broad humps in the higher wavelength region at ∼355 and 548 nm. Surprisingly, in the less polar media (ca. in acetonitrile and in ethanol solvent), the humps ∼355 and 548 nm disappear and the broad but intense band ∼250 nm distinctly splits into two bands one at ∼216/205 and the other at ∼265/259 nm in acetonitrile/ethanol solvent. The intense 250 nm absorption band recorded in aqueous solution undergoes a considerable red shift with a decrease in polarity of the solvent. This result may presage that the peak of 10944


B2 ← 1A1 B2 ← 1A1 1

0.068 0.019

5. CONCLUSION The adsorption behavior of the biologically and industrially significant 2-mercaptoimidazole (2-MI) molecules adsorbed on the nanocolloidal silver surface has been investigated by SERS aided by density functional theory. The optimized structural parameters of all the different forms of the molecule have been estimated from the above-mentioned level of theory. The vibrational modes of the molecule have been assigned for the first time. The results of complete vibrational analyses of the molecule indicate that 43% of the normal, 25% of the cationic, 21% of the ylidic, and 11% the anionic forms of the molecule are prevalent in aqueous solution at neutral pH. Moreover, SERS spectra recorded at various concentrations of the adsorbate at neutral pH medium reveal the concomitance of all the forms of the molecule in the surface adsorbed state. The surface selection rule together with the rigorous vibrational analyses of the molecule suggest the edge-on and face on adsorptive stances of the normal, ylidic and the anionic, cationic forms of the molecule, respectively. The selective enhancement of Raman bands in the SERS spectra of the molecule has been unveiled from the view of the Albretcht’s “A” and Herzberg−Teller (HT) charge transfer (CT) contribution.

232 A′ ← 1A′ 1

2MI+ symmetry (Cs)

0.9358

1

A′ ←1A′

240 348

1

symmetry (C2v)

bands of nontotally symmetric normal modes at ∼610 (A2 for anionic, A″ for cationic) and 771 cm−1 (B1 for anionic, A″ for cationic) emanating from the anionic and/or cationic forms of the molecule may be due to significant involvement of Albrecht’s C term and the rest from the EM contribution due to the coupling of the localized surface plasmons.

0.051 244 A′ ← 1A′ A′ ← 1A′ 0.11 0.011

The experimentally observed FTIR and NRS band and theoretically simulated vibrational frequencies of the cationic, normal, ylidic, and the anionic forms of the molecule in the gas phase are shown in Table S1 along with their tentative assignments. The FTIR spectra of powdered molecule in KBr pellet and the corresponding NRS spectrum of the molecule in neat solid are shown in panels a and b of Figure S1, respectively. Figure S2 shows the bar diagram indicating the relative population of the cationic, normal, ylidic, and anionic forms of the molecule in aqueous solution at neutral pH. This information is available free of charge via the Internet at http://pubs.acs.org.

1

2-MI ylide

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AUTHOR INFORMATION

Corresponding Author

233 358

1

ASSOCIATED CONTENT

S Supporting Information *

symmetry (Cs)

oscillator strength (f)

Article

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*J.C.: tel/fax, +91-33-2462-6869; e-mail, joydeep72_c@ rediffmail.com. G.B.T.: tel, +91-33-24734971; fax, +91-3324732805; e-mail,[email protected]. Notes

The authors declare no competing financial interest.

250 355

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ACKNOWLEDGMENTS We thank DST, Government of India (Project No.-SR/S2/ CMP-0079/2010(G)), for partial financial support. J.C. thanks the DAE-BRNS, Government of India, for financial support through the research project (Project No: 2012/37P/27/BRNS/ ). S.C. and J.C. are grateful to the authorities of IACS for providing the facilities needed to carry out the research work.

π → π*

2-MI experimental (in aqueous solution) transition

Table 3. Experimental and Theoretical Absorption Maxima [λmax (nm)] of the Molecule

calculated


symmetry (Cs)

2MI−



10945



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