Structural dynamics of 4‐cyanobenzaldehyde in S2(ππ*) state

The excited state structural dynamics of 4‐cyanobenzaldehyde (p‐CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A‐ and B‐band absorptions were, respectively, assigned to the major n_O → π₃* and π₂ → π₃* transitions according to the B3LYP‐TD/6‐31G(d) and CIS/6‐31G(d) computations, and the resonance Raman spectra. It was determined that the actual S₂(π₂π₃) state was in energy lower than S₃(π₁π₃), which was just opposite to the B3LYP‐TD/6‐31G(d) calculated order of the S₂(π₁π₃) and S₃(π₂π₃). The vibrational assignments were carried out for the A‐ and B‐band resonance Raman spectra. The B‐band resonance Raman intensities of p‐CNB were dominated by the C2–C3/C5–C6 symmetric stretch mode ν₈, the overtones nν₈ and their combination bands with the ring C–H bend mode ν₁₇, the C9–N10 stretch mode ν₆, the C7–O8 stretch mode ν₇ and the remaining modes. The conical intersection between S₁(n_Oπ₃) and S₂(π₂π₃) states of p‐CNB was determined at complete active space self‐consistent field (CASSCF)(8,7)/6‐311G(d,p) level of theory. The B‐band short‐time structural dynamics and the corresponding decay dynamics of p‐CNB were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S₁(n_Oπ₃)/S₂(π₁π₂π₃π₄)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S_{2, FC}(π₂π₃) to the S₁(n_Oπ₃) state was proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Decay Dynamics of 3‐methyl‐3‐pentene‐2‐one from the light absorbing S2(ππ*) state ‐ Resonance Raman Spectroscopy and CASSCF Study

The photophysics of 3‐methyl‐3‐pentene‐2‐one (3M3P2O) after excitation to the S₂(ππ*) electronic state were studied using the resonance Raman spectroscopy and complete active space self‐consistent field (CASSCF) method calculations. The A‐band resonance Raman spectra were obtained in cyclohexane, acetonitrile, and methanol with excitation wavelengths in resonance with the first intense absorption band to probe the structural dynamics of 3M3P2O. The B3LYP‐TD/6‐31++G(d, p) computation was carried out to determine the relative A‐band resonance Raman intensities of the fundamental modes, and the result was used to reproduce the corresponding fundamental band intensities of the 223.1 nm resonance Raman spectrum and thus to examine whether the vibronic‐coupling existed in Franck‐Condon region or not. CASSCF calculations were carried out to determine the minimal singlet excitation energies of S_{1, FC}, S_1,min (nπ*), S_{2, FC}, S_2,min (ππ*), the transition energies of the conical intersection points S_n/S_π, S_n/S₀, and the optimized excited state geometries as well as the geometry structures of the conical intersection points. The A‐band short‐time structural dynamics and the corresponding decay dynamics of 3M3P2O were obtained by the analysis of the resonance Raman intensity pattern and CASSCF computations. It was revealed that the initial structural dynamics of 3M3P2O was towards the simultaneous C₃=C₄ and C₂=O₇ bond elongation, with the C₃=C₄ bond length lengthening greater at the very beginning, whereas the C₂=O₇ bond length changing greater at the later evolution time before reaching the CI(S₂/S₁) conical intersection point. The decay dynamics from S₂(ππ*) to S₁(nπ*) via S₂(ππ*)/S₁(nπ*) in singlet realm and from S₁(nπ*) to T₁(nπ*) via ISC[S₁(nπ*)/T₂(ππ*)/T₁(nπ*)] in triplet realm are proposed. Copyright © 2014 John Wiley & Sons, Ltd.     

Excited‐state structural dynamics and vibronic coupling of 1,3‐dithiole‐2‐thione—resonance Raman spectroscopy and density functional theory calculation study

1,3‐Dithiole‐2‐thione (DTT) was synthesized and characterized using NMR, FT‐Raman, FT‐IR, UV spectroscopies. Resonance Raman spectra (RRs) were obtained with 341.5, 354.7 and 368.9 nm excitation wavelengths and density functional calculations were done to elucidate the electronic transitions and the RRs of DTT in cyclohexane solution. The RRs indicate that the Franck‐Condon region photodynamics is predominantly along the CS stretch+ H‐CC‐H scissor υ₄, accompanied by the H‐CC‐H scissor υ₃, S‐C‐S symmetric stretch υ₆, CC stretch υ₂, and overtone of the non‐totally symmetric SC‐S2 out‐of‐plane deformation 2υ₁₁. The excited‐state dynamics and the force constant of CS stretch calculated by the RRs were discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Solvent‐dependent structural dynamics of 2(1H)‐pyridinone in light absorbing S4(ππ*) state

The B‐band resonance Raman spectra of 2(1H)‐pyridinone (NHP) in water and acetonitrile were obtained, and their intensity patterns were found to be significantly different. To explore the underlying excited state tautomeric reaction mechanisms of NHP in water and acetonitrile, the vibrational analysis was carried out for NHP, 2(1D)‐pyridinone (NDP), NHP–(H₂O)_n (n = 1, 2) clusters, and NDP–(D₂O)_n (n = 1, 2) clusters on the basis of the FT‐Raman experiments, the B3LYP/6‐311++G(d,p) computations using PCM solvent model, and the normal mode analysis. Good agreements between experimental and theoretically predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands in both the FT‐Raman and the resonance Raman spectra. The results indicated that most of the B‐band resonance Raman spectra in H₂O was assignable to the fundamental, overtones, and combination bands of about ten vibration modes of ring‐type NHP–(H₂O)₂ cluster, while most of the B‐band resonance Raman spectra in CH₃CN was assigned to the fundamental, overtones, and combination bands of about eight vibration modes of linear‐type NHP–CH₃CN. The solvent effect of the excited state enol‐keto tautomeric reaction mechanisms was explored on the basis of the significant difference in the short‐time structural dynamics of NHP in H₂O and CH₃CN. The inter‐molecular and intra‐molecular ESPT reaction mechanisms were proposed respectively to explain the Franck–Condon region structural dynamics of NHP in H₂O and CH₃CN.Copyright © 2015 John Wiley & Sons, Ltd.     

Structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state

The short‐time structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S₂(ππ*) state were studied by using resonance Raman spectroscopy and quantum mechanical calculations. The vibrational spectra and ultraviolet absorption spectra of 4‐formaldehyde imidazole were assigned. The resonance Raman spectra of imidazole and 4‐formaldehyde imidazole were obtained in methanol and acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the short‐time structural dynamics. complete active space self‐consistent field calculations were carried out to determine the minimal singlet excitation energies and structures of S₁(nπ*), S₂(ππ*), and conical intersection point S₁(nπ*)/S₂(ππ*). The results show that the A‐band structural dynamics of imidazole is predominantly along the N₁H/C₄H/C₅H/C₂H in‐plane bending reaction coordinate, which suggests that excited state proton or hydrogen transfer reaction takes place somewhere nearby the Franck–Condon region. The significant difference in the short‐time structural dynamics between 4‐formaldehyde imidazole and imidazole is observed, and the underlying mechanism is interpreted in term of excited state charge redistribution. Copyright © 2015 John Wiley & Sons, Ltd.     

Resonance Raman spectra and excited state structural dynamics of ethylene trithiocarbonate in the A‐ and B‐band absorptions

A‐ and B‐band resonance Raman spectra were acquired for ethylene trithiocarbonate in cyclohexane solution. The results indicate that the S₃ state structural dynamics is mostly along vibrational motions of the CS stretch υ₁₁, while the S₄ state one has motions mainly via the S C S symmetric stretch υ₁₈, CS stretch υ₁₁, and the H C H rock + S C S antisymmetric stretch υ14 reaction coordinates. The very different excited state structural dynamics were briefly discussed in terms of vibronic couplings using local symmetry point group. Copyright © 2009 John Wiley & Sons, Ltd.     

Resonance Raman spectroscopic and density functional theory investigation of the excited state structural dynamics of 2‐mercapto‐1‐methylimidazole

The resonance Raman spectroscopy in conjunction with the density functional theory calculations were used to study the excited state structural dynamics of 2‐mercapto‐1‐methylimidazole (MMI). The experimental UV absorption bands were assigned according to the time‐dependent density functional calculations. The vibrational assignments were done for the A‐band resonance Raman spectra of MMI in water and acetonitrile on the basis of the Fourier transform infrared (FT‐IR) and FT‐Raman measurements in solid, in water and in acetonitrile and the corresponding B3LYP/6‐311+G(d, p) computations. The dynamic structures of MMI were obtained by analysis of the resonance Raman intensity pattern and normal mode analysis. The differences in the dynamic structures of MMI and thiourea were revealed and explained. The structural dynamic of MMI was found to be similar to that of 2‐thiopyrimidone in terms of major reaction coordinates and thus favored the intra‐molecular proton transfer reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Absorption and resonance Raman spectroscopy of the 1Bu state of trans‐1,3,5‐hexatriene including vibronic coupling

The vibronic coupling between the first excited S₁ (2¹A_g) and the second excited S₂ (1¹B_u) singlet electronic states in spectroscopy of trans‐1,3,5‐hexatriene molecule is investigated on the basis of a model consisting of two electronic states coupled by two vibrational modes. Employing a perturbation theory that treats the intramolecular couplings in a perturbative manner, the absorption and resonance Raman cross sections and excitation profiles of this molecule are calculated using the time‐correlation function formalism. The non‐Condon corrections are included in evaluation of cross sections. The multidimensional time‐domain integrals that arise in these calculations have been evaluated for the case in which S₀ (1¹A_g) S₂ (1¹B_u) electronic transition takes place between displaced and distorted harmonic potential energy surfaces. The calculated spectra are in good agreement with the experimental ones. Copyright © 2011 John Wiley & Sons, Ltd.     

The resonance and pre‐resonance Raman scattering in the low energy 11B2u and 11B3u states of 1,4,5,8‐naphthalenetetracarboxy dianhydride in terms of DFT methods. The aggregation of NTCDA molecules in the ethanol and methanol solutions

The absorption, pre‐resonance, and resonance Raman spectra of 1,4,5,8‐naphthalenetetracarboxy dianhydride (NTCDA) molecule are examined in terms of time‐dependent density functional theory at TD‐B3LYP/aug‐cc‐pVDZ and TD‐SVWN/aug‐cc‐pVDZ levels. The Franck–Condon analysis performed for two 1¹A_g → 1¹B_2u and 1¹A_g → 1¹B_3u overlapping transitions leads to an excellent agreement between the theoretical predictions and experimental data providing that the calculated absorption is confronted with that measured in chloroform or acetonitrile but not in the ethanol (methanol) solution. We argue that absorption spectra measured in ethanol solution already known in literature do not characterize NTCDA molecule. On the basis of the absorption and resonance scattering spectra, we may deliberate that the dimerization of NTCDA is responsible for utterly different absorption observed in ethanol (methanol) comparing with less active solvents (chloroform and acetonitrile). That conclusion is supported by the dimer vibronic theory and by the TD‐B97d computations at the aug‐cc‐pVDZ basis set level. Copyright © 2013 John Wiley & Sons, Ltd.     

Short‐time photodecay dynamics of meso‐tetra(p‐hydroxyphenyl)porphine in the condensed phase explored via resonance Raman spectroscopy and density functional theory calculation

Resonance Raman (RR) spectra of free‐base meso‐tetra(p‐hydroxyphenyl)porphine(THPP) were obtained with 397.9, 416 and 514 nm excitation wavelengths, and density functional calculations were carried out to help the elucidation of the photorelaxation dynamics of Soret (B_x and B_y bands) electronic transitions and the RR spectra of THPP. The RR spectrum indicates that the Franck–Condon (FC) region photorelaxation dynamics for the S₀ → S₅ excited electronic state is predominantly along the totally symmetric C_m phC stretching and the C_βC_β stretchingand simultaneously along the asymmetric (C_mC_α)as stretching, ν(phC  C)asstretching, δ(NH)s and γ(C_βH) vibrational relaxation processes, while that for S₀ → S₄ electronic state is predominantly along the C_m phC stretching and pyrrole breathing. The excited‐state structural dynamics of THPP determined from the RR spectra shows that internal conversion (IC) B_y → B_x electronic relaxation occurs in tens of femtoseconds, and the short‐time dynamics is interpreted using the time‐dependent wave packet theory and Herzberg–Teller (vibronic coupling) contributions. Copyright © 2010 John Wiley & Sons, Ltd.     

Resonance Raman intensity analysisof the excited‐state photochemical dynamicsof dimethyl 1,3‐dithiole‐2‐thione‐4,5‐dicarboxylate in the A‐band absorption

Dimethyl 1,3‐dithiole‐2‐thione‐4,5‐dicarboxylate (DDTD) was synthesized and characterized using NMR, Fourier transform (FT)‐Raman, Fourier transform‐infrared (FT‐IR) and UV spectroscopies. Resonance Raman spectra (RRs) were obtained with 341.5, 354.7 and 368.9 nm excitation wavelengths and density functional calculations were carried out to elucidate the π (S C S) →π* (S C S) electronic transitions and the RRs of DDTD in cyclohexane solution. The RRs indicate that the Franck–Condon region photo dynamics have a multidimensional character with motion predominantly along the CS stretch and the C S symmetric stretch modes in the five‐member heterocycle. A preliminary resonance Raman intensity analysis was carried out and the results for DDTD were compared with previously reported results for 1,3‐dithiole‐2‐thione (DTT). Differences and similarities of the spectra in terms of molecular symmetry and electron density are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

A‐band structural dynamics of thioanisole by resonance Raman spectroscopy

The structural dynamics of thioanisole in the S₂(ππ*) electronic state that has large oscillator strength was studied by using the resonance Raman spectroscopy. The vibrational assignments were done for thioanisole on the basis of the FT‐Raman and FT‐IR measurements, the density‐functional theory computations and the normal mode analysis. The A‐ and B‐band resonance Raman spectra were obtained in cyclohexane, methanol and acetonitrile, in which ten modes in A’ irreducible representations were observed. The structural dynamics were obtained according to the resonance Raman intensity pattern. The vibroinc‐coupling between the S₃(πσ*) electronic state that has no oscillator strength and the S₂(ππ*) electronic state were revealed. We discuss the correlation between our present structural dynamics and the previous S₂(ππ*)/S₃(πσ*) conical intersectional dynamics revealed by resonant‐enhanced two‐photon ionization and the photofragment excitation spectroscopic study. Copyright © 2012 John Wiley & Sons, Ltd.     

The excited state dynamics study of di‐2‐pyridylketone in the A‐band and B‐band absorptions by using resonance Raman spectroscopy,IR and UV–visible spectroscopy

Excitation wavelengths of 282.4, 273.9 (A band), 252.7, 239.5 and 228.7 nm (B band) resonance Raman spectra were acquired for di‐2‐pyridylketone, and density functional calculations were carried out to help in the elucidation of the photo relaxation dynamics of A‐band and B‐band electronic transitions. The resonance Raman spectra show that the intensity pattern of the A band presents great difference from that of the B band, which indicate that the short‐time A‐band (S₀→S₄) photo relaxation dynamics have substantial difference from that of B band (S₀→S₁₀) . The overall picture of short‐time dynamics and the vibronic coupling mechanisms are interpreted using Albrecht's theory. Copyright © 2012 John Wiley & Sons, Ltd.     

TDDFT study on the excited‐state hydrogen bonding of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide in methanol solution

The time‐dependent density functional theory method was performed to investigate the excited‐state hydrogen‐bonding dynamics of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide (2a) and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide (3a) in methanol (meoh) solution. The ground and excited‐state geometry optimizations, electronic excitation energies, and corresponding oscillation strengths of the low‐lying electronically excited states for the complexes 2a + 2meoh and 3a + 2meoh as well as their monomers 2a and 3a were calculated by density functional theory and time‐dependent density functional theory methods, respectively. We demonstrated that the three intermolecular hydrogen bonds of 2a + 2meoh and 3a + 2meoh are strengthened after excitation to the S₁ state, and thus induce electronic spectral redshift. Moreover, the electronic excitation energies of the hydrogen‐bonded complexes in S₁ state are correspondingly decreased compared with those of their corresponding monomer 2a and 3a. In addition, the intramolecular charge transfer of the S₁ state for complexes 2a + 2meoh and 3a + 2meoh were theoretically investigated by analysis of molecular orbital. Copyright © 2014 John Wiley & Sons, Ltd.     

Time‐resolved Raman studies on Al2O3: Cr3+: lifetime measurements of the excited‐state transition Ē → 2Ā

A time‐resolved intensified charge coupled device‐based Raman microspectrometer system dedicated to the study of solid samples is described, offering good optical, temporal and spatial resolution. The advantages of this approach are demonstrated on Al₂O₃:Cr³⁺, obtaining for the first time the temporal evolution of the excited state transition Ē → 2Ā. Moreover, the time dependence of the luminescence due to the chromium ion was also determined by the same Raman device. Copyright © 2010 John Wiley & Sons, Ltd.     

Observation of persistent α‐helical content and discrete types of backbone disorder during a molten globule to ordered peptide transition via deep‐UV resonance Raman spectroscopy

The molten globule (MG) state can aid in the folding of a protein to a functional structure and is loosely defined as an increase in structural disorder with conservation of the ensemble secondary structure content. Simultaneous observation of persistent secondary structure content with increased disorder has remained experimentally problematic. As a consequence, modeling how the MG state remains stable and how it facilitates proper folding remains difficult due to a lack of amenable spectroscopic techniques to characterize this class of partially unfolded proteins. Previously, deep‐UV resonance Raman (dUVRR) spectroscopy has proven useful in the resolution of global and local structural fluctuations in the secondary structure of proteins. In this work, dUVRR was employed to study the MG to ordered transition of a model four‐helix bundle protein, HP7. Both the average ensemble secondary structure and types of local disorder were monitored, without perturbation of the solvent, pH, or temperature. The MG to ordered transition is induced by stepwise coordination of two heme molecules. Persistent dUVRR spectral features in the amide III region at 1295–1301 and 1335–1338 cm⁻¹ confirm previous observations that HP7 remains predominantly helical in the MG versus the fully ordered state. Additionally, these spectra represent the first demonstration of conserved helical content in a MG protein. With successive heme binding, significant losses are observed in the spectral intensity of the amide III₃ and S regions (1230–1260 and 1390 cm⁻¹, respectively), which are known to be sensitive to local disorder. These observations indicate that there is a decrease in the structural populations able to explore various extended conformations with successive heme binding events. DUVRR spectra indicate that the first heme coordination between two helical segments diminishes exploration of more elongated backbone structural conformations in the inter‐helical regions. A second heme coordination by the remaining two helices further restricts protein motion. Copyright © 2013 John Wiley & Sons, Ltd.     

Ultraviolet resonance Raman spectroscopy of a β‐sheet peptide: a model for membrane protein folding

The partitioning of a hydrophobic hexapeptide, N‐acetyl‐tryptophan‐pentaleucine (AcWL5), into self‐associated β‐sheets within a vesicle membrane was studied as a model for integral membrane protein folding and insertion via vibrational and electronic spectroscopy. Ultraviolet resonance Raman spectroscopy allows selective examination of the structures of amino acid side chains and the peptide backbone and provides information about local environment and molecular conformation. The secondary structure of AcWL5 within a vesicle membrane was investigated using 207.5‐nm excitation and found to consist of β‐sheets, in agreement with previous studies. The β‐sheet peptide shows enhanced Raman scattering cross‐sections for all amide modes as well as extensive hydrogen‐bonding networks. Tryptophan vibrational structure was probed using 230‐nm excitation. Increases in Raman cross‐sections of tryptophan modes W1, W3, W7, W10, W16, W17, and W18 of membrane‐incorporated AcWL5 are primarily attributed to greater resonance enhancement with the B_b electronic transition. The W17 mode, however, undergoes a much greater enhancement than is expected for a simple resonance effect, and this observation is discussed in terms of hydrogen bonding of the indole ring in a hydrophobic environment. The observed tryptophan mode frequencies and intensities overall support a hydrophobic environment for the indole ring within a vesicle, and these results have implications for the location of tryptophan in membrane protein systems. Copyright © 2009 John Wiley & Sons, Ltd.     

UV resonance Raman study of TrpZip2 and related peptides: π‐π interactions of tryptophan

Aromatic interactions are important stabilizing forces in proteins but are difficult to detect in the absence of high‐resolution structures. Ultraviolet resonance Raman spectroscopy is used to probe the vibrational signatures of aromatic interactions in TrpZip2, a synthetic β‐hairpin peptide that is stabilized by edge‐to‐face and face‐to‐face tryptophan π‐π interactions. The vibrational markers of isolated edge‐to‐face π‐π interactions are investigated in the related β‐hairpin peptide W2W11. The bands that comprise the Fermi doublet exhibit systematic shifts in position and intensity for TrpZip2 and W2W11 relative to the model peptide, W2W9, which does not form aromatic interactions. Additionally, hypochromism of the B_b absorption band of tryptophan in TrpZip2 leads to a decrease in the relative Raman cross‐sections of B_b‐coupled Raman bands. These results reveal spectral markers for stabilizing tryptophan π‐π interactions and indicate that ultraviolet resonance Raman may be an important tool for the characterization of these biological forces. Copyright © 2012 John Wiley & Sons, Ltd.     

The intramolecular charge transfer in a donor–π–acceptor dianion probed by resonance Raman spectroscopy and quantum chemical calculations

The dideprotonation of 4‐(4‐nitrophenylazo)resorcinol generates an anionic species with substantial electronic π delocalization. As compared to the parent neutral species, the anionic first excited electronic transition, characterized as an intramolecular charge transfer (ICT) from the CO⁻ groups to the NO₂ moiety, shows a drastic red shift of ca. 200 nm in the λ_max in the UV‐vis spectrum, leading to one of the lowest ICT energies observed (λ_max = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push‐pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (ε_max) in comparison to the neutral species is observed. The resonance Raman enhancement profiles reveal that in the neutral species the chromophore involves several modes, as ν(C N), ν(NN), ν(CC) and ν_s(NO₂), whereas in the dianion, there is a selective enhancement of the NO₂ vibrational modes. The quantum chemical calculations of the electronic transitions and vibrational wavenumbers led to a consistent analysis of the enhancement patterns observed in the resonance Raman spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer of 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone in methanol: A TD‐DFT study

Excited‐state intermolecular or intramolecular proton transfer (ESIPT) reaction has important potential applications in biological probes. In this paper, the effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer reaction of the 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone (MQ) dye in methanol solvent is investigated by the density functional theory and time‐dependent density functional theory approaches. Both the primary structure parameters and infrared vibrational spectra analysis of MQ and its benzo‐analogue 2‐methyl‐3‐hydroxy‐4(1H)‐benzo‐quinolone (MBQ) show that the intermolecular hydrogen bond O₁―H₂?O₃ significantly strengthens in the excited state, whereas another intermolecular hydrogen bond O₃―H₄?O₅ weakens slightly. Simulated electron absorption and fluorescence spectra are agreement with the experimental data. The noncovalent interaction analysis displays that the intermolecular hydrogen bonds of MQ are obviously stronger than that of MBQ. Additionally, the energy profile analysis via the proton transfer reaction pathway illustrates that the ESIPT reaction of MBQ is relatively harder than that of MQ. Therefore, the effect of benzo‐annelation of the MQ dye weakens the intermolecular hydrogen bond and relatively inhibits the proton transfer reaction.