Structure and Vibrational Spectra of Mononitrated Benzo [a] Pyrenes.

The molecules benzo[a]pyrene (BaP) and 1-, 3-, and 6-nitrobenzo[a]pyrene (1-NBaP, 3-NBaP, 6-NBaP) are currently of significant interest due to their presence in respirable combustion exhaust particulates and their mutagenic and carcinogenic properties. Structure−function correlations as well as spectroscopic signatures for trace analysis are necessary for these benzo[a]pyrene derivatives. In this paper, detailed infrared and Raman spectroscopic data of BaP and its three mononitrated isomers are provided for the first time. By utilizing density functional theory (DFT, B3LYP method with 6-311+G** basis set), the molecular geometries and the vibrational spectra are calculated. Good agreement is noted between the calculated and experimental geometry for BaP, and predictions of the vibrational data for all compounds are within ∼5 cm-1 of the experimental data. Normal mode assignments are proposed with particular emphasis on the nitro group vibrations. The geometrical distortions of the BaP structure upon nitro group substitution and correlations between structural parameters and vibrational data as well as structure−function relationships related to the mutagenicity of this important class of polycyclic aromatic hydrocarbons are discussed

Pinpointing The Extent Of Electronic Delocalization In The Re(i)-to-tetrazine Charge-separated Excited State Using Time-resolved Infrared Spectroscopy

Femtosecond mid-IR transient absorption spectroscopy (TRIR) and time-dependent density functional theory (TD-DFT) calculations on Re(CO)(3)Cl(Me(2)BPTZ) [Me(2)BPTZ = 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine] are used to demonstrate that the lowest excited state of the complex is a triplet metal-to-ligand charge-transfer ((3)MLCT) state with a lifetime of 225 ps. The short excited-state lifetime is explained by the energy-gap taw. Vibrational cooling of the (3)MLCT state shows up as early-time dynamics (3.6 ps). The structural changes in the excited state are deduced from the frequency shifts in the TRIR vibrational bands. The vibrational frequencies of the CO groups increase upon excitation as a result of decreased back-bonding between the CO ligands and the oxidized Re center in the (3)MLCT state. The vibrational frequencies of the central tetrazine ring of Me(2)BPTZ decrease because of the decrease in the bond order upon reduction of the Me(2)BPTZ ligand in the (3)MLCT state. Interestingly, the TRIR signals from the pyridine moieties of Me2BPTZ were not detected. These results can be explained by localization of the electronic charge on the central tetrazine ring in the (3)MLCT state of Re(CO)(3)Cl(Me(2)BPTZ), as supported by TD-DFT calculations

Supplementary data for article: Ruan, Y.; Taha, H. A.; Yoder, R. J.; Maslak, V.; Hadad, C. M.; Bađić, J. D. The Prospect of Selective Recognition of Nerve Agents with Modular Basket-like Hosts. A Structure-Activity Study of the Entrapment of a Series of Organophosphonates in Aqueous Media. Journal of Physical Chemistry. Part B: Condensed Matter, Materials, Surfaces, Interfaces and Biophys 2013, 117 (11), 3240–3249. https://doi.org/10.1021/jp401841w

Supporting information for: [https://doi.org/10.1021/jp401841w]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/1619

Mechanism Of N(5)-ethyl-flavinium Cation Formation Upon Electrochemical Oxidation Of N(5)-ethyl-4a-hydroxyflavin Pseudobase

We investigated the oxidation behavior of 5-ethyl-4a-hydroxy-3-methy1-4a,5-dihydrolumitlavin (pseudobase Et-FlH) in acetonitrile with the aim of determining if the two-electron oxidized Et-FlOH(2+) undergoes a release of hydroxyl cation and the production of 5-ethyl-3methyllumiflavinium cation (Et-Fl(+)). The focus of this work is to investigate the possibility of using Et-FlOH as a catalyst for water oxidation. The cyclic voltammetry demonstrates that Et-FlOH exhibits two one-electron oxidation potentials at +0.95 and +1.4 V versus normal hydrogen electrode (NHE), with the second oxidation potential being irreversible. The production of Et-FY\u27 is observed in the cyclic voltammetry of Et-FlOH and has been previously assigned to the release of OH(+) from the two-electron oxidized Et-FlOH(2+). The results of our study show that this is not the case: (i) we performed bulk electrolysis of the electrolyte solution at +2 V and then added Et-FlOH to the electrolyzed solution. We found that Et-Fr is produced from this solution, even though Et-FlOH itself was not oxidized; (ii) reactions of Et-FlOH with chemical oxidants (eerie ammonium nitrate, nitrosyl tetrafluoroborate, and tetrabutylammonium persulfate) demonstrate. that Et-Fl(+) production occurs only in the presence of strong Lewis acids, such as Ce(4+) and NO(+) ions. On the basis of these results, we propose that the production of Et-Fl+ in the electrochemistry of Et-FlOH(-1) occurs because of the shift in the Et-FlOH/Et-Fl+ acid base equilibrium in the presence of protons released during anodic oxidation. We identified two sources of protons: (i) oxidation of traces of water present in the acetonitrile releases oxygen and protons and (ii) two-electron oxidized Et-FlOH(2+) releases protons located on the N(5)-alkyl chain. The release of protons from Et-FlOH(2+) was confirmed by cyclic voltammetry of Et-FlOH in the presence of pyridine as a base. The first oxidation peak of Et-FlOH at +0.95 V is reversible in the absence of pyridine. The addition of pyridine leads to the shift of the oxidation potential to a less positive value, which is consistent with a proton-coupled electron transfer (PCET). Furthermore, the anodic current increases, and the cathodic peak becomes irreversible, giving rise to two additional reduction peaks at -0.2 and -1 V. The same reduction peaks were observed in the high scan rate cyclic voltammogram of Et-FlOH in the absence of pyridine, implying that the release of protons indeed occurs from Et-FlOH(2+). To determine which functional group of Et-FlOH(center dot+) is the source of protons, we performed DFT calculations at the B3LYP/6-311++G level of theory for a reaction of Et-FlOH(center dot+). with pyridine and identified two proton sources: (i) the \u3eN-CH(2)(-) group of the N(5) alkyl chain and (ii) the OH group in the 4a-position of the radical cation. Because the appearance of new reduction peaks at 0.2 and 1.0 V occurs in the model compound that lacks OH protons (Et-FlOMe), we conclude that the proton removal occurs predominantly from the \u3eN-CH(2)- moiety

Supplementary data for article: Chen, S.; Ruan, Y.; Brown, J. D.; Gallucci, J.; Maslak, V.; Hadad, C. M.; Bađić, J. D. Assembly of Amphiphilic Baskets into Stimuli-Responsive Vesicles. Developing a Strategy for the Detection of Organophosphorus Chemical Nerve Agents. Journal of the American Chemical Society 2013, 135 (40), 14964–14967. https://doi.org/10.1021/ja408585j

Supporting information for: [https://doi.org/10.1021/ja408585j]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/1427

From Selection to Instruction and Back: Competing Conformational Selection and Induced Fit Pathways in Abiotic Hosts

Two limiting cases of molecular recognition, induced fit (IF) and conformational selection (CS), play a central role in allosteric regulation of natural systems. The IF paradigm states that a substrate “instructs” the host to change its shape after complexation, while CS asserts that a guest “selects” the optimal fit from an ensemble of preexisting host conformations. With no studies that quantitatively address the interplay of two limiting pathways in abiotic systems, we herein and for the first time describe the way by which twisted capsule M-1, encompassing two conformers M-1(+) and M-1(−), trap CX4 (X=Cl, Br) to give CX4⊂M-1(+) and CX4⊂M-1(−), with all four states being in thermal equilibrium. With the assistance of 2D EXSY, we found that CBr4 would, at its lower concentrations, bind M-1 via a M-1(+)→M-1(−)→CBr4⊂M-1(−) pathway corresponding to conformational selection. For M-1 complexing CCl4 though, data from 2D EXSY measurements and 1D NMR line-shape analysis suggested that lower CCl4 concentrations would favor CS while the IF pathway prevailed at higher proportions of the guest. Since CS and IF are not mutually exclusive, we reason that our work sets the stage for characterizing the dynamics of a wide range of already existing hosts to broaden our fundamental understanding of their action. The objective is to master the way in which encapsulation takes place for designing novel and allosteric sequestering agents, catalysts and chemosensors akin to those found in nature

Electronic Properties Of N(5)-ethyl Flavinium Ion

We investigated the electronic properties of N(5)-ethyl flavinium perchlorate (Et-Fl(+)) and compared them to those of its parent compound, 3-methyllumiflavin (Fl). Absorption and fluorescence spectra of Fl and Et-Fl(+) exhibit similar spectral features, but the absorption energy of Et-Fl(+) is substantially lower than that of Fl. We calculated the absorption signatures of Fl and Et-Fl(+) using time-dependent density functional theory (TD-DFT) methods and found that the main absorption bands of Fl and Et-Fl(+) are (pi,pi*) transitions for the S(1) and S(3) excited states. Furthermore, calculations predict that the S(2) state has (n,pi*) character. Using cyclic voltammetry and a simplistic consideration of the orbital energies, we compared the HOMO/LUMO energies of Fl and Et-Fl(+). We found that both HOMO and LUMO orbitals of Et-Fl(+) are stabilized relative to those in Fl, although the stabilization of the LUMO level was more pronounced. Visible and mid-IR pump-probe experiments demonstrate that Et-Fl(+) exhibits a shorter excited-state lifetime (590 ps) relative to that of Fl (several nanoseconds), possibly due to faster thermal deactivation in Et-Fl(+), as dictated by the energy gap law. Furthermore, we observed a fast (23-30 ps) S(2) -\u3e S(0) internal conversion in transient absorption spectra of both Fl and Et-Fl(+) in experiments that utilized pump excitations with higher energy

Mechanistic Study Of The Photochemical Hydroxide Ion Release From 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine

The excited-state behavior of 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine and its derivative, 9-methoxy-10-methyl-9-phenyl-9,10-dihydroacridine (AcrOR, R = H, Me), was studied via femtosecond and nanosecond UV-vis transient absorption spectroscopy. The solvent effects on C-O bond cleavage were clearly identified: a fast heterolytic cleavage (tau = 108 ps) was observed in protic solvents, while intersystem crossing was observed in aprotic solvents. Fast heterolysis generates 10methyl-9-phenylacridinium (Acr(+)) and -OH, which have a long recombination lifetime (no signal decay was observed within 100 mu s). AcrOH exhibits the characteristic behavior needed for its utilization as a chromophore in the pOH jump experiment

Supplementary data for article: Ruan, Y.; Taha, H. A.; Yoder, R. J.; Maslak, V.; Hadad, C. M.; Bađić, J. D. The Prospect of Selective Recognition of Nerve Agents with Modular Basket-like Hosts. A Structure-Activity Study of the Entrapment of a Series of Organophosphonates in Aqueous Media. Journal of Physical Chemistry. Part B: Condensed Matter, Materials, Surfaces, Interfaces and Biophys 2013, 117 (11), 3240–3249. https://doi.org/10.1021/jp401841w

Supporting information for: [https://doi.org/10.1021/jp401841w]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/1619

Supplementary data for article: Chen, S.; Ruan, Y.; Brown, J. D.; Gallucci, J.; Maslak, V.; Hadad, C. M.; Bađić, J. D. Assembly of Amphiphilic Baskets into Stimuli-Responsive Vesicles. Developing a Strategy for the Detection of Organophosphorus Chemical Nerve Agents. Journal of the American Chemical Society 2013, 135 (40), 14964–14967. https://doi.org/10.1021/ja408585j

Supporting information for: [https://doi.org/10.1021/ja408585j]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/1427