Microwave spectra, molecular structure, and aromatic character of 4a,8a-azaboranaphthalene

Abstract

The microwave spectra for seven unique isotopologues of 4a,8a-azaboranaphthalene [hereafter referred to as BN-naphthalene] were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with (10)B, (11)B, and all unique single (13)C and the (15)N isotopologue (with (11)B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the (11)B(14)N isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, DJ = 0.06(1) kHz, 1.5χaa ((14)N) = 2.5781(61) MHz, 0.25(χbb - χcc) ((14)N) = - 0.1185(17) MHz, 1.5χaa (11B) = - 3.9221(75) MHz, and 0.25(χbb - χcc) ((11)B) = - 0.9069(24) MHz. The experimental inertial defect is Δ = - 0.159 amu Å(2), which is consistent with a planar structure for the molecule. The B-N bond length from the experimentally determined structure is 1.47 Å, which indicates π-bonding character between the B and N. The measured quadrupole coupling strengths provide important and useful information about the bonding, orbital occupancy, and aromatic character for this aromatic molecule. Extended Townes-Dailey analyses were used to determine the B and N electron sp(2)-hybridized and p-orbital occupations. These results are compared with electron orbital occupations from the natural bond orbital option in theoretical calculations. From the analyses, it was determined that BN-naphthalene has aromatic character similar to that of other N-containing aromatics. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene. Accurate and precise structural parameters were obtained from the microwave measurements on seven isotopologues and from high-level G09 calculations.

Synthesis, microwave spectra, x-ray structure, and high-level theoretical calculations for formamidinium formate

“…26 The inertial defect for the tropolone -formic acid complex 10 constants is ∆ = -1.46 amu Å 2 and = 1.33 amu Å 2 for the propiolic acid -formic acid complex, 4 and for B-N naphthalene( 11 B 14 N), ∆ = -0.159 amu Å 2 . 27…”

Mono- and Dinitro-BN-Naphthalenes: Formation and Characterization

“…The bond lengths of 4 , 6 , and 7 are summarized in Table 2 , along with the reported bond lengths of the parent BNN [ 39 , 40 ]. All other crystal structure data can be found in the supporting information.…”

“…Due to the disorder of the molecule, the boron-carbon and nitrogen-carbon bond lengths are averaged. c Ref [40], the data were obtained from a microwave measurement. 6, 1977408; and 7, 1977409. b Ref.…”

Six-Membered Rings With Two or More Heteroatoms With at Least One Boron

“…Authors predict that the extent of the π-electron delocalization of the three isomeric azaborines would be slightly affected by CH replacement with nitrogen, meaning that molecular properties associated with aromaticity would more or less resemble those of parent azaborines. 18 Following the monocyclic BN substituted benzenes, electronic properties of a variety of BN-aromatics, including BN-naphthalenes [19][20][21][22] , BN-phenanthrenes [23][24][25][26] , BN-anthracenes [26][27][28] , BN-pyrenes 13,29,30 , aryl-fused bis-BN dihydropyrenes 31 , bis-BN-ullazine derivatives 32 , BN-helicenes 33 , BN-coronenes and nanographene [34][35][36][37][38] have been studied using theoretical methods. The relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure.…”

“…The λ max of the lower energy band in the UV-vis absorption spectrum is as follows: anthracene, 356 nm; BN-Anth, 359 nm; bis-BN-Anth, 357 nm. 27 Microwave spectra and quadrupole coupling constants were reported for 1-ethyl-1,2-dihydro-1,2-azaborine, 55 1,2-dihydro-1,2-azaborine, 56 and 4a,8a-azaboranaphthalene 21 . Results of the microwave measurements and calculations provided clear evidence for the aromatic character of BN heteroaromatics.…”