Identification of two Ag‐2,2′:6′,2″‐terpyridine surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis: evidence for chemical mechanism contribution to SERS of Ag(0)–tpy

Measurement and interpretation of the excitation wavelength dependence of surface‐enhanced Raman scattering (SERS) spectra of molecules chemisorbed on plasmonic, e.g. Ag nanoparticle (NP) surfaces, are of principal importance for revealing the charge transfer (CT) mechanism contribution to the overall SERS enhancement. SERS spectra, their excitation wavelength dependence in the 445–780‐nm range and factor analysis (FA) were used for the identification of two Ag‐2,2′:6′,2″‐terpyridine (tpy) surface species, denoted Ag⁺–tpy and Ag(0)–tpy, on Ag NPs in systems with unmodified and/or purposefully modified Ag NPs originating from hydroxylamine hydrochloride‐reduced hydrosols. Ag⁺–tpy is a spectral analogue of [Ag(tpy)]⁺ complex cation, and its SERS shows virtually no excitation wavelength dependence. By contrast, SERS of Ag(0)–tpy surface complex generated upon chloride‐induced compact aggregate formation and/or in strongly reducing ambient shows a pronounced excitation wavelength dependence attributed to a CT resonance (the chemical mechanism) contribution to the overall SERS enhancement. Both the resonance (λ_exc = 532 nm) and off‐resonance (λ_exc = 780 nm) pure‐component spectra of Ag(0)–tpy obtained by FA are largely similar to surface‐enhanced resonance Raman scattering (λ_exc = 532 nm in resonance with singlet metal to ligand CT (¹ MLCT) transition) and SERS (λ_exc = 780 nm) spectra of [Fe(tpy)₂]²⁺ complex dication. Copyright © 2014 John Wiley & Sons, Ltd.     

Detection of a new SRS‐promoting phonon mode and cross‐cascaded χ(3)‐nonlinear lasing in single crystals of calcite (trigonal CaCO3)

For calcite (CaCO₃), one of the pioneer crystals in nonlinear optics, new results of stimulated Raman scattering (SRS) spectroscopy are presented. Among them are the discovery of a new SRS‐promoting vibration mode with ω_SRS2 ≈︁ 282 cm^‐1 and its participation, together with the main SRS mode ω_SRS1 ≈︁ 1086.5 cm^‐1, in cross‐cascaded (χ⁽³⁾ ↔ χ⁽³⁾) nonlinear‐lasing generation, as well as the observation of efficient self‐upconversion via cascaded parametric four‐wave processes of one‐micron Stokes and anti‐Stokes χ⁽³⁾‐lasing into the UV‐region of third harmonic generation. The investigations show that calcite is able to generate a χ⁽³⁾‐lasing comb of more than two octaves bandwidth. The article also gives a brief review on the discovery and study of the SRS‐effect in natural crystals (minerals), which have expanded our ability to study the photon‐phonon nonlinear‐laser interactions in crystalline materials. A short summary of information about χ⁽³⁾‐lasing properties of the triangular planar structure units in SRS‐active crystals is included.     

Solvatochromism of 2‐(N,N‐dimethylamino)‐7‐nitrofluorene and the natural dye β‐carotene: application for the determination of solvent dipolarity and polarizability

The effects of solvents on chemical phenomena (rate and equilibrium constants, spectroscopic transitions, etc.) are conveniently described by solvation free‐energy relationships that take into account solvent acidity, basicity and dipolarity/polarizability. The latter can be separated into its components by manipulating the UV–vis spectra of two solvatochromic probes, 2‐(N,N‐dimethylamino)‐7‐nitrofluorene (DMANF) and a di‐(tert‐butyl)‐tetramethyl docosanonaen probe (ttbP9) whose synthesis is laborious and expensive. Recently, we have shown that the natural dye β‐carotene can be conveniently employed instead of ttbP9 for the determination of solvent polarizability (SP) of 76 molecular solvents and four ionic liquids. In the present work, we report the polarizabilities of further 24 solvents. Based on the solvatochromism of β‐carotene and DMANF, we have calculated solvent dipolarity (SD) for 103 protic and aprotic molecular solvents, and ionic liquids. The dependence of SD and SP on the number of carbon atoms in the acyl‐ or alkyl group of several homologous series (alcohols; 2‐alkoxyethanols; carboxylic acid‐ anhydrides, and esters, ionic liquids) is calculated and briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 4‐substituted(X) ‐ cub‐1‐yl and ‐bicyclo[2.2.2]oct‐1‐yl cations: a DFT study

The effects of substituents on the stability of 4‐substituted(X) cub‐1‐yl cations ( 2 ), as well as the benchmark 4‐substituted(X) bicyclo[2.2.2]oct‐1‐yl cation systems ( 7 ), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) level of theory. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 2 / 7 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values of both systems are best described by a combination of both substituent constants. This highlights the distinction between through‐space and through‐bond electronic influences characterized by σ_F and σ_χ, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis of novel bis(β‐cyclodextrin)s linked with aromatic diamine and their molecular recognition with model substrates

Novel β‐cyclodextrin (β‐CD) dimers with aromatic diamine linkers, 1,3‐(aminomethyl)‐benzylamine‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (2) , 4,4′‐diaminodiphenylmethano‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (3) , and 4,4′‐ ethylenedianiline‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (4) , were synthesized. The inclusion complexation behaviors of these compounds, together with 4,4′‐aminophenyl ethyl‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (5) , with substrates such as acridine red (AR), neutral red (NR), ammonium 8‐anilino‐1‐naphthalenesulfonate (ANS), sodium 2‐(p‐toluidinyl) naphthalenesulfonate (TNS), rhodamine B (RhB), and brilliant green (BG), were investigated by ultraviolet, fluorescence, circular dichroism, and 2D NMR spectroscopy. The results indicated that the two linked CD units cooperatively bound to a guest, and the molecular binding affinity toward substrates, especially curved guest ANS and linear guests such as NR and AR, was increased. The linker length between two CD units played a crucial role in the molecular recognition of the hosts with guest dyes. The binding constants of the hosts for AR, TNS, ANS, and RhB decreased with increasing linker length in hosts 2‐4 . Moreover, structurally similar hosts 3 and 5 exhibited very different binding behavior for the guests. Host 5 showed much higher Ks values toward positively charged guests and lower Ks toward negatively charged guests than host 3 . The 2D NMR spectra of hosts 3 and 5 with RhB were acquired to understand the binding difference between 3 and 5 . The molecular binding ability and selectivity of model substrates by these hosts were sufficiently investigated to reveal not only the cooperative contributions of the linker group and CD cavities upon inclusion complexation with dye guest molecules, but also the controlling factors for the molecular selective binding. Copyright © 2008 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations: a DFT study

The effects of substituents on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations (3) and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations (4), for a set of substituents (X = H, NO₂, CN, NC, CF₃, CHO, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH₃⁺) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) and B3LYP/6‐31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σ_χ dependency of charge delocalization in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

¹⁷O NMR shieldings of 3‐substituted(X)bicyclo[1.1.1]pentan‐1‐ols ( 1 , Y = OH), 4‐substituted(X)bicyclo[2.2.2]octan‐1‐ols ( 2 , Y = OH), 4‐substituted(X)‐bicyclo[2.2.1]heptan‐1‐ols ( 3 , Y = OH), 4‐substituted(X)‐cuban‐1‐ols ( 4 , Y = OH) and exo‐ and endo‐ 6‐substituted(X)exo‐bicyclo[2.2.1]heptan‐2‐ols ( 5 and 6 , Y = OH, respectively), as well as their conjugate bases ( 1 – 6 , Y = O^?), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH, F, Cl, OH, NH₂, CH₃, SiMe₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model at the B3LYP/6‐311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁷O SCS and polar field and group electronegativity substituent constants (σ_F and σ_χ, respectively) and also the NBO derived molecular parameters (oxygen natural charge, Q_n, occupation numbers of the oxygen lone pairs, n_o, and occupancy of the C? O antibonding orbital, σ*_CO(occup)). In the case of the alcohols ( 1 – 6 , Y = OH) the ¹⁷O SCS appear to be governed predominantly by the σ_χ effect of the substituent. Furthermore, the key determining NBO parameters appear to be n_o and σ*_CO(occup). Unlike the alcohols, the calculated ¹⁷O SCS of the conjugate bases ( 1 – 6 , Y = O^?), except for system 1 , do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1 (Y = O^?) raises a significant conundrum with respect to their origin. Copyright © 2010 John Wiley & Sons, Ltd.     

Gas phase interaction of L‐proline with Be2+, Mg2+ and Ca2+ ions: a computational study

Geometry optimisation and metal ion affinities (MIAs) of the binding configurations of Be²⁺, Mg²⁺ and Ca²⁺ to L ‐proline were calculated using the hybrid Density Functional Theory (DFT‐B3LYP) and second order Møllet?Plesset perturbation theory (MP2) methods. Be²⁺ was found to bind preferentially in a charge transfer type arrangement through the carbonyl oxygen (? C?O) and the lone pair of the imino‐group nitrogen atom (? NH? ). On the contrary Mg²⁺ and Ca²⁺ were found to prefer binding in a bi‐dentate manner through the carboxylate group of L ‐proline (OCO) in a zwitterion form. The main types of interactions found to influence the binding preference of M²⁺ ions to L ‐proline were (i) charge transfer in the case of Be²⁺ ions and (ii) electrostatic interactions in the case of Mg²⁺ and Ca²⁺ ions. Inspection of the IR stretching of the N? H and the O? H groups of L ‐proline with M²⁺ ions in a chelating configuration (to both O and N atoms) indicated a considerable shift to higher frequency with decreasing MIA. On the other hand, the MIA for the zwitterion L ‐proline with M²⁺ tracks the reciprocal distance of the M²⁺? OCO bond further confirming that the nature of the bond is mainly electrostatic. Comparison with other molecules containing the carboxylic function is also included in order to gain more insight on the types of interaction of this amino acid with metal ions in general. Copyright © 2007 John Wiley & Sons, Ltd.     

The effect of the pH on the interaction of L‐arginine with colloidal silver nanoparticles. A Raman and SERS study

Raman and surface enhanced Raman scattering (SERS) spectroscopies were used to study the pH effect (7 to 9) on the interaction of arginine (Arg) with colloidal Ag nanoparticles (AgNps). A new methodology was implemented in order to obtain reproducible SERS spectra in solution. The dependence of the Arg concentration on the stability of the AgNps is discussed. A pH increasing of the colloidal solution to the limits of the Arg pKa₂ value induces a preferential and stable Arg–metal interaction. ξ potential measurements of the Arg–AgNps system at different pH conditions studied provide information about the Arg–AgNps interaction; the pH increasing favors the interaction. SERS spectra at pH 7 indicate that the molecule interacts with the Ag surface only through the guanidinium fragment. By increasing the pH to 9, the molecule adopts a new conformation on the surface; the metal–analyte interaction is verified through the guanidinium, carboxylate and the aliphatic moieties. In addition, theoretical calculations performed by using the extended Hückel method for a model of Arg interacting with an Ag surface support the observed SERS results. Copyright © 2013 John Wiley & Sons, Ltd.