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Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution.

ABSTRACT: Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4'-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4- salt (1) in an acidic aqueous solution. Efficient cleavage of the N-N bond (4 ps) to form the N,N-di(4-bromophenyl)nitrenium ion (DN) was also observed in the acidic aqueous solution. As a result, the dication intermediate 4 appears more likely to be produced after abstracting a proton for the nitrenium ion DN in the acid solution first, followed by an electron abstraction to form the radical cation intermediate 3. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.

SUBMITTER: Du L

PROVIDER: S-EPMC6862224 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution.

Du Lili L Yan Zhiping Z Bai Xueqin X Liang Runhui R Phillips David Lee DL

International journal of molecular sciences 20191105 21

Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4'-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4- salt (1 ...[more]

PMID: 31694248

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Project description:Nitrenium ion species are examined using computational methods (DFT, MP2, coupled-cluster, and a composite method, CBS-APNO) with a particular emphasis on nonaromatic species (i.e., those lacking an aromatic or heteroaromatic ring in direct conjugation with the formal nitrenium ion center.) The substitution of the N+ center with alkyl, alkoxy, vinyl, acyl, and sulfonyl, among others, was evaluated. For these species, three properties are considered. (1) The stability of the nitrenium ions to unimolecular isomerizations such as 1,2 alkyl or H shifts; to the extent that the singlet states could be characterized as discrete minima on the potential energy surface (PES), (2) the effect of the substituents on singlet-triplet energy splitting as well as (3) the relative stabilities of the nitrenium ions as defined by N-hydration enthalpies (RR'N+ + H2O ? RR'NOH2 +). Nearly all simple alkyl and di-alkyl nitrenium ion singlet states are predicted to rearrange without detectable barriers, largely through 1,2 H or alkyl shifts. Methyl and N,N-dimethylnitrenium ion singlet states could be characterized as formal minima on the PES. However, these species show small or insignificant barriers to isomerization. Disubstituted nitrenium ions that include an alkyl group and a conjugating substituent such as alkoxyl, vinyl, or phenyl show meaningful barriers to isomerization and are thus predicted to possess nontrivial lifetimes in solution. Alkyl groups substantially stabilize the singlet state relative to the situation in the parent nitrenium ion NH2 + to the point where the two states are nearly degenerate. Other groups that interact with the nitrenium ion center decrease the ?E st in the order formoyl < vinyl < phenyl < alkoxy ? sulfonyl < cyclopropyl ? cyclobutyl. The latter two substituents interact strongly with the (singlet) nitrenium ion center through the formation of a nonclassical bonding reminiscent of the bisected cyclopropylcarbinyl ion case for carbocations. When singlet-state stability is evaluated in the context of N-hydration enthalpies, it is found that the ordering is acyl < vinyl < alkoxyl < phenyl < cyclopropyl and cyclobutyl.

Project description:The hydration of the alkali metal ions in aqueous solution has been studied by large angle X-ray scattering (LAXS) and double difference infrared spectroscopy (DDIR). The structures of the dimethyl sulfoxide solvated alkali metal ions in solution have been determined to support the studies in aqueous solution. The results of the LAXS and DDIR measurements show that the sodium, potassium, rubidium and cesium ions all are weakly hydrated with only a single shell of water molecules. The smaller lithium ion is more strongly hydrated, most probably with a second hydration shell present. The influence of the rubidium and cesium ions on the water structure was found to be very weak, and it was not possible to quantify this effect in a reliable way due to insufficient separation of the O-D stretching bands of partially deuterated water bound to these metal ions and the O-D stretching bands of the bulk water. Aqueous solutions of sodium, potassium and cesium iodide and cesium and lithium hydroxide have been studied by LAXS and M-O bond distances have been determined fairly accurately except for lithium. However, the number of water molecules binding to the alkali metal ions is very difficult to determine from the LAXS measurements as the number of distances and the temperature factor are strongly correlated. A thorough analysis of M-O bond distances in solid alkali metal compounds with ligands binding through oxygen has been made from available structure databases. There is relatively strong correlation between M-O bond distances and coordination numbers also for the alkali metal ions even though the M-O interactions are weak and the number of complexes of potassium, rubidium and cesium with well-defined coordination geometry is very small. The mean M-O bond distance in the hydrated sodium, potassium, rubidium and cesium ions in aqueous solution have been determined to be 2.43(2), 2.81(1), 2.98(1) and 3.07(1) Å, which corresponds to six-, seven-, eight- and eight-coordination. These coordination numbers are supported by the linear relationship of the hydration enthalpies and the M-O bond distances. This correlation indicates that the hydrated lithium ion is four-coordinate in aqueous solution. New ionic radii are proposed for four- and six-coordinate lithium(I), 0.60 and 0.79 Å, respectively, as well as for five- and six-coordinate sodium(I), 1.02 and 1.07 Å, respectively. The ionic radii for six- and seven-coordinate K(+), 1.38 and 1.46 Å, respectively, and eight-coordinate Rb(+) and Cs(+), 1.64 and 1.73 Å, respectively, are confirmed from previous studies. The M-O bond distances in dimethyl sulfoxide solvated sodium, potassium, rubidium and cesium ions in solution are very similar to those observed in aqueous solution.

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Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution.

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Time-Resolved Spectroscopic Study of <i>N,N</i>-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution.

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