The iminoborane (HBNH) molecule selectively breaks a B═N bond of smaller diameter single-wall BN nanotubes (BNNTs) and expands a ring at their surface, either at the edges or at the middle of the tube. Density functional theory is used to test whether its organic counterpart HCCH can do the same with BNNTs. HCCH–BNNT complexes are identified and transition states located for these combination reactions. Also explored are possible reactions of HBNH with single-wall carbon nanotubes (SWNTs) and HCCH with SWNTs. Data suggest that B═N (C═C) bond breaking, followed by ring expansion at the surface, may be possible. Although [2 + 2]cycloaddition reaction seems possible for HBNH–BNNTs, a high energy barrier hinders the process for other combinations of reactants. Introduction of substituents to HBNH/HCCH may allow a facile process. In most cases of HCCH–BNNTs, HBNH–SWNTs, and HCCH–SWNTs, transition states are identified and suggest an electron-rich reactant might lower barrier heights to form stable complexes. Reaction with HCCH or HBNH at the bay region of smaller diameter armchair tube is not favorable.

The effect of various solvents on the structures and properties of carboxylated SWNTs has been explored using the same level different basis set approach (SLDB), where B3LYP functional of density functional theory (DFT) was applied. Armchair (4,4) and zigzag (8,0) and (9,0) tubes were considered as the testbed. In order to simulate varying concentration of −COOH groups, one to five acids groups were placed at one end of these tubes. These samples were placed in different solvents (namely, CS2, THF, and water) with varying polarity and results were compared with gas-phase properties. Similar to the gas-phase, zigzag tubes also exhibit both regular (r-COOH, ν(C═O) above 1700 cm–1) and low-frequency (lf-COOH, ν(C═O) below 1700 cm–1) acid groups. Characteristics of the r-COOH group are not affected much in solvents, but lf-COOH of the zigzag tube is the one that makes these tubes distinguishable from its armchair cousin. Stability and charge distribution of SWNT-COOH strongly depend on the number of acid groups in different solvents which may help controlling further functionalization. Vibrational analyses reveal certain features in the 1400–1600 cm–1 range that are characteristic of lf-COOH and in different solvents, which may help in the assignment of experimental spectra of oxidized SWNT in solvents.

Structure and properties of [8]BN-circulenes are calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Structurally they are similar to [8]circulenes but exhibit different electronic and optical properties. For example, carbon circulenes emit in the visible region while BN-circulenes exhibit higher emission energies which fall in the UV region. Tuning of molecular properties was examined by derivatization of [8]BN-circulenes, analogous to that in tetraoxa and tetrathio derivatives. Such materials may be used in optical devices and serve as an alternative to organic light-emitting devices (OLEDs). Absorption in the UV region by [8]BN-circulenes suggests that such compounds may be an excellent candidate for UV-light protection.

A new feature of reactivity of iminoborane (HBNH) at the surface of boron nitride nanotube has been revealed by this theoretical study. The HBNH molecule not only selectively breaks the B═N double bond of the BN nanotubes (BNNTs) but also expands the hexagonal network of the tube to larger cages at the surface. Such expanded structures are stabilizing by 30–50 kcal/mol depending on the chirality and reactive site of the tubes. Complexation energy decreases with diameter (because of the strain effect) of the tube and is lowest for a planar BN-sheet. However, a [2+2]-cycloaddition reaction that is common for iminoborane is also exhibited depending on the site and chirality. For zigzag tube, diagonal BN bonds, either at the edge or at the middle of the tube, are cleaved, but BN bonds parallel to the tube axis undergo cycloaddition reactions. In contrast, diagonal BN bonds of the armchair BN-tube prefer cycloaddition and bonds perpendicular to the tube axis follow this new reactivity pattern. Transition states of both reaction processes have been identified, and the low barrier height (>14 kcal/mol) suggests a bond cleavage and ring-expansion process is slightly more favorable kinetically. Intrinsic reaction coordinate study suggests that an approaching HBNH molecule first forms a cycloaddition product, which in some cases undergoes bond cleavage then ring expansion. Infrared spectra exhibit new very weak peaks which may be helpful in characterizing both bond-cleavage–ring-expansion and cycloaddition products. These findings suggest that BN nanotubes can be used as a carrier of different derivatives of iminoborane and that a wide range of new materials can be developed. Also, low-temperature matrix isolation techniques may be avoided to study R–BN–R′ molecules by attaching to the BN tube surface.

Vibrational frequency analyses using density functional theory (DFT) resolves some structural features of purified oxidized single-wall carbon nanotubes (o-SWNTs). Both COOH and phenolic OH (OHph) groups, predicted in several experimental studies to be present in o-SWNTs, were considered at the tips of armchair and zigzag tubes with varying diameters. Hydrogen bonding, where carbonyl oxygen acts as proton acceptor while phenolic OH donates the proton, leads to the most stable isomers, with a H-bond energy of 9–12 kcal/mol, almost double that of simpler systems. Vibrational frequencies of participating bonds are significantly red-shifted, which is not reflected in experimental spectra, and which leads to the conclusion that phenolic OH is likely not present at the tips of o-SWNTs.

•Construct a new pair-potential functional form for diatomic systems.•Successfully apply it to describe the ground-state van der Waals systems.•Reach a good accuracy for all the range of the internuclear distance.•All the systems show a single binding-energy relation in the attractive region.•Suggest a unique function form for describing the potentials of diatomic systems.A three-parameter pair-potential model recently constructed is improved in the short- and long-range interaction regions. We demonstrate that this improved potential function is able to accurately describe the entire potentials of the ground-state van der Waals systems such as rare-gas, triplet H2, Alkali-helium, Alkaline-earth, and group 12 diatomic systems. All these systems show a single binding-energy relation in the attractive region. The improved function suggests a unique form for describing the potentials of diatomic systems ranging from strongly-bound to weakly-bound diatomic systems.Ground-state helium dimer: comparison of potential energies. Between new model, three-parameter model (XG2005), Tang–Toennies Model (TT1984), Tang–Toennies–Yiu model (TTY1995), LM2M2 (AS1991), variational LM2 (LM1989), quantum Monte Carlo (ATB1993), and experiment.

Energetics of carboxyl groups at the periphery of a graphene sheet are studied using density functional theory (B3LYP) with a 6-31G* basis set, augmented with diffuse functions on O. Corner sites are energetically preferred followed by zigzag edges, and armchair edges are least stable. The energy and geometry of each is attributed to a competition between π-conjugation and steric repulsion factors. Vibrational analyses reveal certain features that are characteristic of each site location, which may help in the assignment of experimental spectra of graphene and other polycyclic aromatic hydrocarbons. For example, zigzag sites typically lead to an intense C═O stretching band that occurs below 1700 cm–1, quite uncommon for the carboxyl group.

Vibrational frequency analysis using density functional theory resolves some ambiguities in interpretation of IR spectra of carboxylated single-wall carbon nanotubes (o-SWNTs). Armchair (n,n) and zigzag (m,0) tubes, with diameter varying between 0.5 and 1.4 nm, were populated with different numbers of COOH groups at one terminus. While armchair-COOH tubes exhibit C═O stretching frequencies in the standard range of 1720–1760 cm–1, zigzag-COOH tubes display a new C═O band around 1650 cm–1 with much higher intensity, in addition to the higher-frequency C═O mode. The COOH groups exhibiting this low-frequency (lf-COOH) are displaced well off the lines of the tube walls, to the outside. This new band is in the same range as one observed experimentally, which has on occasion been attributed to quinone formation. The C═O bond length in the relevant COOH groups is longer by about 0.01 Å than regular C═O bonds. Such low-frequency C═O band is unusual and not common for any stand-alone COOH group and seems characteristic of carboxylated zigzag tubes irrespective of their diameter and the number of acid groups at the terminus. The lower frequency is not a result of H-bonding, nor can it be reproduced by small models such as benzoic acid or extended carbon networks as in graphene-COOH. Its origin is attributed instead to the curvature of the zigzag tubes in addition to their structural arrangement.

Calculations of the full structure and spectra of large nanotubes can be very demanding of computer resources. The advantages and limitations of the cost-effective same level different basis (SLDB) and selected normal modes (SNM) protocols are elucidated for carboxylated (4,4) armchair and (8,0) zigzag single-wall carbon nanotubes (SWNTs) with varying numbers of COOH groups on the tips of the tubes. While armchair-COOH tubes exhibit C═O stretching frequencies in the standard range of 1720–1760 cm–1, zigzag-COOH tubes display a surprising C═O band around 1660 cm–1 with much higher intensity, in addition to the higher-frequency C═O mode. This low-frequency C═O peak is very unusual for a standalone COOH group and is a fingerprint of zigzag tubes.

Vibrational frequencies of functionalized groups (−COOH, −CONH2, and −COOCH3) of tip-modified single-wall carbon nanotubes are estimated using density functional theory. Both metallic (5,5) and semiconducting (10,0) nanotubes are considered with single and multiple functional groups at their tip. Several differences in frequency and intensity of the characteristic C═O band between (5,5) and (10,0) tubes are observed, which might help experimentalists to identify different tubes. For example, (5,5) tubes exhibit higher C═O frequencies than (10,0) tubes for all groups, and these bands are more intense in the latter tubes. These differences persist within a narrow range of diameter. To understand the effect of nanotubes on the spectra, fragment models containing parts of tube attached to functional groups are also studied. Such a computationally inexpensive model (compared to full tube) faithfully reproduces IR spectra and may be used for a wide range of end-modified tubes.

The magnitude and nature of interactions between small aromatic systems (benzene and naphthalene) and various single-wall carbon nanotubes are examined by MP2 theory. π−π stacking configurations are more strongly bound than CH---π analogues. There is a small preference for placement of the aromatic directly above a C═C bond center in the nanotube. All of these complexes are dominated by dispersion forces. Mobility of adsorbed benzene on the tube surface is considered in terms of rotating, tilting, and sliding. As noted previously for covalent modification of nanotubes, the computationally efficient same level different basis set protocol is reliable for studying noncovalent interactions. Previously reported DFT (LDA or GGA) binding energies for π−π stacking arrangements are underestimated, whereas dispersion-corrected methods overestimate these binding energies.

High level electronic structure calculations predict the molecular mechanism of H2 elimination in the LiH + NH3 → LiNH2 + H2 reaction. Geometries and energies of stationary points are obtained using second-order perturbation theory (MP2) and coupled cluster CCSD(T) theory with the aug-cc-pVTZ basis set. The reaction proceeds via intermediate molecular LiNH4 complexes, namely HLiNH3 (reactant side) and LiNH2H2 (product side). Each reactant contributes one H atom to the product H2.