Reservation Energy Bonds and Structural Stability of Series of Multihydrated (<i>n</i>_H_₂_O = 1−10) Glycine−H⁺M⁺ (M = Li, Na, or K) Complexes

A systematic study with the density functional theory (DFT/B3LYP) on the reservation energy bonds (REBs) and structural stability of series of biologically relevant multihydrated (nH2O = 1−10) glycine−H+M+ (M = Li, Na, or K) complexes in the gas phase has been presented. The results show that the bonds (REBs) of (H2O)n-x-glycine−H+...M+(H2O)x (for x = 0−4, x n) can release energy if they are broken. Calculations confirm that each of these REBs can be yielded without suffering any energy barrier only when the carboxyl oxygen of the glycine molecule is first bound by the metal ion M+ (or its hydrate) and then its amino nitrogen is protonated. Stepwise multiple hydrations can reduce charge distributions on the glycine−H+M+ gradually and, thus, increase the binding strength of REBs and favor the stability of the entire hydrated system. Consequently, the effects of reservation energy of these REBs are weakened. However, their energy decreases are not limitless. The lower limit of decrease, being not less than 20.0 kcal/mol for these complexes, is first predicted in the paper. In other words, the formation of a similar REB in a biological system can reserve energy of ≥20 kcal/mol. All the dissociation free energies are more than their corresponding electronic dissociation energies. Different from those dissociation energies, calculations show that these dissociation free energies change less with the addition of attached water molecules. All these reveal that two free reactants in a hydrate system finally become a regular complex and involve more energy in the new complex. These are very consistent with the real biological phenomenon. The small activation energy barrier (<3.0 kcal/mol) in the process of cleavage of the REB is also revealed in the present study, which also is in good agreement with behavior of a real biological system. Correspondingly, the affinity strength of the latter attached water molecules becomes weaker and weaker with further hydration, however, ultimately showing a tendency toward invariableness. The ordering of affinity strengths between these different ions (or ion hydrates) and its corresponding ligand is Li−nW > Na−nW > K−nW (where n = 1−10 and W represents a water molecule), whereas the magnitude ordering of REB energies of these different metal-ion−involved glycine−H+ hydrates reverses. In general, these different M+-involved complexes do not present very large differences in regard to the effect on reservation energy; however, they behave differently in coupling water molecules, using their various active sites. For example, only a water molecule can be attached to the Na+ end of the glycine−H+Na+ complex (GHNa) to form the most-stable Na−W1 monohydrate among its isomers, whereas the most-stable K−W1 monohydrate is generated by attaching a water molecule to the frontal amino hydrogen of the glycine−H+K+ complex (GHK). Comparisons of the relative energies of these Li+/Na+-involved glycine−H+ hydrates indicate that the carboxyl moiety that is coupling the M+ of the glycine is preferred to be hydrated, whereas the relative energies of the isomers of the K+-involved glycine−H+ hydrates imply that the protonated amino moiety of the glycine is the optimal combination site

Cation-Modulated Electron-Transfer Channel: H-Atom Transfer vs Proton-Coupled Electron Transfer with a Variable Electron-Transfer Channel in Acylamide Units

The mechanism of proton transfer (PT)/electron transfer (ET) in acylamide units was explored theoretically using density functional theory in a representative model (a cyclic coupling mode between formamide and the N-dehydrogenated formamidic radical, FF). In FF, PT/ET normally occurs via a seven-center cyclic proton-coupled electron transfer (PCET) mechanism with a N→N PT and an O→O ET. However, when different hydrated metal ions are bound to the two oxygen sites of FF, the PT/ET mechanism may significantly change. In addition to their inhibition of PT/ET rate, the hydrated metal ions can effectively regulate the FF PT/ET cooperative mechanism to produce a single pathway hydrogen atom transfer (HAT) or a flexible proton coupled electron transfer (PCET) mechanism by changing the ET channel. The regulation essentially originates from the change in the O···O bond strength in the transition state, subject to the binding ability of the hydrated metal ions. In general, the high valent metal ions and those with large binding energies can promote HAT, and the low valent metal ions and those with small binding energies favor PCET. Hydration may reduce the Lewis acidity of cations, and thus favor PCET. Good correlations among the binding energies, barrier heights, spin density distributions, O···O contacts, and hydrated metal ion properties have been found, which can be used to interpret the transition in the PT/ET mechanism. These findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide useful information for a greater understanding of PT/ET cooperative mechanisms, and a possible method for switching conductance in nanoelectronic devices

Hydrogen-Bonding-Assisted Substituent Engineering for Modulating Magnetic Spin Couplings and Switching in <i>m</i>‑Phenylene Nitroxide Diradicals

Rational modification of the coupler for the theoretical design of molecular magnets has attracted extensive interest. Substituent insertion is a widely used strategy for adjusting molecular properties, but its effect and modulation on magnetic spin couplings have been less investigated. In this work, we predict the magnetic properties of the design m-phenylene nitroxide (NO) diradicals regulated by introducing substituents. The calculated results for those two pairs of diradicals indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably change after substituent regulation in the range from 253 to 730 cm–1. Such noticeable magnetic changes induced by introducing subsituents are mainly attributed to different electronic effects of substituents, assisted by the proximity of two NO groups, good planarity, conjugation, and an intramolecular hydrogen bond. In particular, the insertion of intramolecular H-bonds not only indicates an electronic effect but also has greatly changed the spin density distribution. Further aromaticity of the coupler ring, spin densities, and molecular orbitals and energetics was evaluated to gain a better understanding of magnetic regulation. Interestingly, further protonation of some substituents (e.g., −NO2 and −CO2) can noticeably turn the spin coupling from ferromagnetic to antiferromagnetic, showing manipulable magnetic switching. This work provides a promising strategy based on substituent engineering for magnetic spin coupling modulation, not only turning the coupling magnitude but also enabling the magnetic switching, thus providing insights into molecular magnetic manipulation for spintronics applications

Single- versus Multi-Proton-Coupled Rydberg-State Electron Transfer in Amine Clusters

Amino fragments (−NH₂) are well-known to exist widely in biological systems and their protonated forms are inclined to trap electrons and form Rydberg radicals (−NH₃^•) in the electron-excess systems. Taking CH₃–NH₃⁺ as a mimicking group of the protonated alkylamine side-chain of lysine, ab initio calculations indicate that the proton/electron cooperatively transfer from CH₃NH₃ to CH₃NH₂ via a single-proton-coupled Rydberg-state electron transfer (ET) mechanism with an Rydberg-orbital channel for ET outside the −NH_n hydrogens and a N–H⁺ → N proton migrating pathway. Besides, in big amine clusters, CH₃NH₃·(NH₃)_<i>n</i>·NH₂CH₃ (<i>n</i> = 1–3), the proton/electron transfer along an amine wire is stepwise and every step takes place via the similar single-proton-coupled Rydberg-state ET mechanism with low energy barrier (<4.0 kcal/mol). When a water chain, (H₂O)_<i>n</i> (<i>n</i> = 1–3), lies between CH₃NH₃ and NH₂CH₃ as a bridge, the energy barriers (8.5–15.0 kcal/mol) of proton/electron cooperatively transfer between CH₃NH₃ and NH₂CH₃ are raised significantly as compared to these of the pure amine wires (<4.0 kcal/mol). We attribute this fact to the combined effects of the proton binding energies and electron affinities of CH₃NH₂ and H₂O. Interestingly, different from the amine-wire case, movement of the solvated electron along the water-wire can promote two or three protons synchronously moving at the same direction. This process can be described in terms of a multi-proton-coupled solvated-ET mechanism

Rational Design of Magnetic DNA Motifs with Diradical Character: Nitroxide Functionalization of Nucleobases

In the current work, the nitroxide radical groups are utilized to functionalize the nucleobases, obtaining the nucleobase diradical building blocks for magnetic DNA with significant ferromagnetic or antiferromagnetic coupling characteristics. The nitroxide functionalization strategies include introduction of nitroxide radical group to the carbon site and oxidization of the amino group in nucleobases, and the diradical-functionalized nucleobases are denoted by ^2NOX, where X = A, G, T, and C bases. The density functional theory calculations reveal that these nitroxide diradicalized nucleobases are stable and have large magnetic spin coupling magnitudes. Almost all of them possess antiferromagnetic-like spin coupling characteristics with considerably large spin coupling constants [<i>J</i> = −671.7 (^2NOA1), −463.3 (^2NOA3), −370.5 (^2NOG), −494.9 (^2NOC1), −3265.5 (^2NOT), and −2445.5 cm^–1 (^2NOC3)] expect for ^2NOC2 and ^2NOA2 which have the ferromagnetic-like spin coupling characteristics (<i>J</i> = 149.1 and 440.7 cm^–1), respectively. The spin alternation rule works well for these nitroxide-diradicalized nucleobases in interpreting the magnetic spin coupling characters although such heterocyclic nucleobases (purine and pyrimidine) are as the couplers, and the spin coupling constants present good linear relationships with the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gaps and the energy gaps between the closed-shell singlet and triplet state of these nucleobase diradicals. Besides, their magnetic coupling properties are also analyzed by the shape of the singly occupied molecular orbitals (SOMOs) and SOMO–SOMO energy splitting of the triplet state, the H-bonding with their complementary nucleobases, and the nitroxide radical group orientations. Clearly, this work provides a novel strategy for the rational design of the magnetic DNA motifs with well-defined diradical characters and also provides insights into the spin coupling interactions in these nucleobase-based magnet building blocks of the magnetic DNA nanowires

Enhanced <i>J</i>‑Couplings through Specially Solvated Electron in Perfluoro‑[<i>n</i>]Prismanes and [<i>n</i>]Asteranes

Perfluoro-[n]prismanes ((C2F2)n, n = 3–8) and [n]asteranes ((C3F4)n, n = 3–5) exhibit a strong perfluoro cage effect that can stably encapsulate an additional electron inside the cage. The 2s-like distribution of solvated electron (esol–) not only changes the molecular structure but also affects the nuclear spin properties. In this work, we reveal how the esol– enhances and regulates indirect nuclear spin–spin coupling between two coupled F nuclei (JFF-coupling). Results show that esol– is mainly distributed in the central cavity, and a part of it penetrates into the C-shell and C–F bond regions due to the unique polyhedral C-shell structure. Such a 2s-like esol– creates a novel esol– based coupling mechanism, including the newly generated through-esol– (TSE) and esol–-enhanced traditional through-bonds and through-space (esol–-enhanced TB+TS) pathways, enhancing and regulating N(e)JFF-coupling, which crosses N bonds in the shortest TB pathway and is affected by esol–. The contribution of the TSE (JTSE) is positive and increases with the increase of the central angle between two coupled F nuclei (∠F⊗F), and the contribution of the esol–-enhanced TB+TS (JTB+TS) is negative and |JTB+TS| decreases with the increase of N and straight linear distance between two coupled F nuclei (dFF). Interestingly, N(e)JFF exhibits a special dependence on N/dFF and ∠F⊗F due to the cooperation and competition between JTSE and JTB+TS. When ∠F⊗F sol–-enhanced TB+TS can play a role; JTB+TS determines sign and magnitude of N(e)JFF. When ∠F⊗F > 70°, the TSE dominates, and JTSE determines sign and magnitude of N(e)JFF. This work not only further enriches information on the states, distributions, and properties of esol– but also provides insights into the nuclei spin properties in perfluorinated polyhedrons triggered by esol–

Multiwater-Assisted Proton Transfer Study in Glycinamide Using Density Functional Theory

Multiwater-assisted proton transfers (PTs) involving two and three water molecules from the amide nitrogen to carbonyl oxygen atom in model peptide compound glycinamide have been investigated employing the B3LYP/6-311++G** level of theory. The thermodynamic and kinetic parameters, such as tautomeric energies, equilibrium constants, barrier heights, and rate constants, have been predicted, respectively. The relevant quantities associated with the proton-transfer processes, such as geometrical structures, interaction energies, and intrinsic reaction coordinate (IRC) calculations, have also been studied. In addition, the factors influencing the thermodynamic and kinetic parameters, such as temperature dependences, solvent effects, and deuteration effects, have also been explored qualitatively, respectively. Computational results show that the PT barrier heights are 16.93 (4.98) and 18.95 (6.67) kcal/mol in the forward (reverse) directions with the assistances of two and three water molecules, which are reduced significantly by 28.43 (25.95) and 26.41 (24.26) kcal/mol compared with those of direct intramolecular PT, respectively. Both of the PT processes proceed with a concerted mechanism, reflecting the bifunctional roles of the water, that is, it can accept a proton from the donor site in glycinamide and transfer a different proton to the acceptor site in glycinamide. The optimal numbers of water molecules directly participating in the PT may be two compared with the barrier heights among the various water-assisted PT cases. Applications of the IPCM model within the framework of the self-consistent reaction filed (SCRF) theory indicate that the bulk solvent has a subtle influence on the thermodynamic and kinetic properties

Photophysical Characters of Rationally Designed Hetero-Ring-Expanded Guanine Analogues and Effect of Cytosine Pairing

We present the results of the CIS and TDB3LYP calculations of the optical absorption and emission spectra of some newly designed guanine (G) analogues and their Watson−Crick base pairs. Compared with natural G, the onset absorption peaks of these newly designed analogues are red-shifted, while the fluorescence peaks are blue-shifted. In general, the first excited singlet states (ππ*) of these analogues are nonplanar for all bases considered here. But, the Stokes shifts for the designed G-analogues are much smaller than that of natural G, suggesting that they have stronger molecular rigidity and higher fluorescence quantum yields than those of natural G. The first excited states of these Watson−Crick base pairs essentially originate from those of their isolated purine moieties, as demonstrated from the S1 geometries of their Watson−Crick base pairs. For G and its analogues, A1 and A2 (they are ring-expanded with one-bond intercalation at the C5 site), the pairing with cytosine reduces the oscillator strengths of both the first absorption peak (by 27%−60%) and the fluorescent emission (by 19%−23%), while for the analogues A3, A4, and xG in which G is ring-expanded with a two-bond intercalation at the C5 site, the pairing, in contrast, increases the oscillator strengths of both the first absorption peak (by 11%−15%) and the fluorescent emission (by 3%−20%). These observations indicate that the pairing with cytosine can quench the fluorescence for G, A1, and A2 but enhance the fluorescence quantum yields for A3, A4, and xG. The significant shifts induced by ring-expansion in the ring-expanded G with a two-bond intercalation at the C5 site reveal a possibility for their fluorescent detections

Intramolecular Proton Transfer Modulation of Magnetic Spin Coupling Interaction in Photochromic Azobenzene Derivatives with an Ortho-Site Hydroxyl as a Modulator

Proton transfer modulation in an organic diradical is apparently the most conspicuously attractive phenomenon. In this work, we have computationally designed the trans and cis forms of photochromic azobenzene- (AB-) bridged diradicals by considering AB as coupler and two nitroxide (NO) as spin sources and a −OH attaching at the ortho site as modulator. Our object is that through intramolecular proton transfer to protonate the azo-unit (−NN−) the magnetic coupling characteristics of the designed diradicals can be modulated in their photocontrolled trans and cis forms. The calculated results indicate that PT can significantly regulate the magnetic spin coupling constants, J = −701.3 cm–1 ↔ −286.2 cm–1 for the trans form and −544.1 cm–1 ↔ −328.1 cm–1 for the cis form. In particular, we discover that these designed magnetic molecules can undergo magnetic conversion between antiferromagnetic and ferromagnetic coupling through PT, besides there is considerable increase in the magnitude of their magnetic coupling constants J, (e.g., −59.97 to 172.4 cm–1) for the trans-mode at the m/m linking site. Moreover, we discover that the nitroxide radicals at different linking positions have a significant impact and remarkably alter the magnetic spin coupling characteristics of AB-based diradicals. Besides, various radical groups are used as spin sources which corroborated our assumptions and tended to the same conclusion. This work offers a novel understanding of the spin interaction mechanism and a viable approach for the rational design of new AB-based magnets which are beneficial for further applications in the future