Quadrupole Coupling Constants Research Articles

High-resolution solid-state NMR study of the effect of composition on network connectivity and structural disorder in multi-component glasses in the diopside and jadeite join: Implications for structure of andesitic melts

The structural evolution of andesitic melts with varying compositions remains one of the unsolved questions in high-temperature geochemistry and petrology. In this article, we report the structural details of model andesitic glasses [CaO–MgO–Na2O–Al2O3–SiO2 (CMNAS)] in the diopside (CaMgSi2O6) and jadeite (NaAlSi2O6) join using high-resolution, multi-nuclear, solid-state nuclear magnetic resonance (NMR). The 27Al NMR spectra of CMNAS glasses confirm that [4]Al is dominant. While a minor fraction of [5]Al is observed, its presence is only prevalent in the glasses with higher Ca–Mg content. Topological disorder in the glass network also tends to increase with Ca–Mg content as evidenced by the increase in the quadrupolar coupling constant (Cq) of [4]Al for glasses with increasing diopside contents (XDiopside). Despite the complex nature of the glasses studied here (with five oxide components), the 17O 3QMAS NMR spectra resolve diverse bridging oxygens (BOs) and non-bridging oxygens (NBOs), from which the degree of Al avoidance among framework cations (Si and Al) and preferential proximity among non-network cations (Ca2+, Mg2+, and Na+) and each oxygen site can be estimated: presence of Al–O–Al in jadeite glass implies a violation of the Al-avoidance rule in the glasses and the decrease in the fraction of NBOs with increasing XDiopside is consistent with a decrease in their viscosity. Analysis of the peak position of {Ca, Mg}-mixed NBOs, along with the absence of Na-NBO peak, and the peak shape of Si–O–Al reveals preferential partitioning of Ca2+ and Mg2 into NBOs and the proximity of Na+ to BOs. The fraction of highly coordinated Al has been linked to thermodynamic and transport properties of the melts. Considering all the experimental Al coordination environments available in the literature, together with the current experimental studies, we attempt to establish the relationship between the fractions of highly coordinated Al and composition, particularly average cationic potential of non-network forming cations (<c/r>ave, defined as cationic potential normalized by the mole fraction of each non-network cation). The fraction of highly coordinated Al increases nonlinearly with increasing <c/r>ave. The fraction of [5,6]Al is negligible up to <c/r>ave=∼1.7, then it increases above <c/r>ave>∼1.7 regardless of changes in other compositional variables (e.g., Si/Al, NBO content). This indicates the presence of a threshold <c/r>ave value for the formation of [5,6]Al. The current experimental results with the changes in network polymerization, coordination environments, and the degree of disorder in the CMNAS glasses can improve understanding of the structure–property (particularly, configurational thermodynamic properties) relationships of multi-component natural silicate melts, including andesitic melts and glasses.

The microwave spectra and molecular structures of 2-chloro-1,1-difluoroethylene and its complex with the argon atom

The microwave spectra of six isotopologues each of 2-chloro-1,1-difluoroethylene and its argon complex are obtained in the 5.6–20.5GHz region using a combination of broadband chirped pulse and Balle–Flygare cavity Fourier transform microwave techniques. The structure of the halogen substituted ethylene has been determined using a combination of theory and experiment. Argon is found to form a nonplanar complex with this haloethylene, locating in the ClCCF cavity, which maximizes its interactions with the heavy atoms in the ethylene subunit. A combined analysis of the chlorine quadrupole coupling constants of the monomer and of the complex allows the sign of the only nonzero off-diagonal component of the quadrupole coupling tensor for the monomer to be determined; thus, the complete tensor for the monomer has been derived.

Temperature-induced amorphization of Na-zeolite A: A view from multi-nuclear high-resolution solid-state NMR

Despite efforts to understand the amorphization mechanisms of zeolites upon heating and subsequent dehydration, little is known about the extent of Si-Al disorder and topological variations in both crystalline and amorphous phases during amorphization. In this study, we investigated the atomic structure and the extent of configurational disorder (e.g., Si-Al ordering) in Na-zeolite A and other dehydrated phases during their temperature-induced amorphization using multi-nuclear solid-state NMR. We also report the first multi-nuclear (17O, 29Si, and 27Al) NMR spectra of the intermediate amorphous phases. 29Si MAS NMR results confirm the prevalence of amorphous phases up to ~1073 K and variation in Q-species for the crystalline phases. The 27Al quadrupolar coupling constant of the [4]Al peak in Na-zeolite A and the intermediate amorphous phases increase with increasing temperature, which suggests an increase in the topological disorder associated with the structural distortion around [4]Al. 2D 17O 3QMAS NMR spectra resolve the crystallographically distinct Si-O-Al sites in Na-zeolite A and three types of oxygen linkages namely, Si-O-Al, Si-O-Si, and Al-O-Al in the intermediate amorphous phases, which provides an unambiguous experimental evidence for an increase in the Si-Al disorder during the amorphization of zeolite. The detailed structural changes in Na-zeolite A and other dehydrated phases at various temperatures provide insights into the structural changes of other aluminosilicates during amorphization, thereby highlighting the changes in Si-Al ordering.

H₂-AgCl: a spectroscopic study of a dihydrogen complex.

H2-AgCl has been observed on a Fourier transform microwave spectrometer equipped with laser ablation source and determined to be a dihydrogen complex. Transitions up to J = 3-2 have been measured and analyzed for four isotopologues of the complex containing ortho and para H2. The ortho and para spin states have been included in one fit, a deviation from the typical H2 complex. Rotational constants B and C, centrifugal distortion constants Δ(J) and Δ(JK), nuclear electric quadrupole coupling constants χ(aa), χ(bb), and χ(cc) for (35)Cl and (37)Cl have been fit for both spin states while nuclear spin-nuclear spin constants D(aa), D(bb), and D(cc), and nuclear spin-rotation constant C(aa) have been reported for the ortho spin state. Quantum chemical calculations predict a strong bonding interaction and the strength of the complex has been related to reported χ(aa) and Δ(J) values amongst a host of comparable species, including the AgCl monomer itself. Bond lengths have been determined for Ag-Cl, Ag-H2 center-of-mass, and H-H and are reported.

Changes in the geometries of C₂H₂ and C₂H₄ on coordination to CuCl revealed by broadband rotational spectroscopy and ab-initio calculations.

The molecular geometries of isolated complexes in which a single molecule of C2H4 or C2H2 is bound to CuCl have been determined through pure rotational spectroscopy and ab-initio calculations. The C2H2···CuCl and C2H4···CuCl complexes are generated through laser vaporization of a copper rod in the presence of a gas sample undergoing supersonic expansion and containing C2H2 (or C2H4), CCl4, and Ar. Results are presented for five isotopologues of C2H2···CuCl and six isotopologues of C2H4···CuCl. Both of these complexes adopt C(2v), T-shaped geometries in which the hydrocarbon binds to the copper atom through its π electrons such that the metal is equidistant from all H atoms. The linear and planar geometries of free C2H2 and C2H4, respectively, are observed to distort significantly on attachment to the CuCl unit, and the various changes are quantified. The ∠(*-C-H) parameter in C2H2 (where * indicates the midpoint of the C≡C bond) is measured to be 192.4(7)° in the r0 geometry of the complex representing a significant change from the linear geometry of the free molecule. This distortion of the linear geometry of C2H2 involves the hydrogen atoms moving away from the copper atom within the complex. Ab-initio calculations at the CCSD(T)(F12*)/AVTZ level predict a dihedral ∠(HCCCu) angle of 96.05° in C2H4···CuCl, and the experimental results are consistent with such a distortion from planarity. The bonds connecting the carbon atoms within each of C2H2 and C2H4, respectively, extend by 0.027 and 0.029 Å relative to the bond lengths in the isolated molecules. Force constants, k(σ), and nuclear quadrupole coupling constants, χ(aa)(Cu), [χ(bb)(Cu) - χ(cc)(Cu)], χ(aa)(Cl), and [χ(bb)(Cl) - χ(cc)(Cl)], are independently determined for all isotopologues of C2H2···CuCl studied and for four isotopologues of C2H4···CuCl.

Microwave spectrum, van der Waals bond length, and 131Xe quadrupole coupling constant of Xe–SO3

Nine isotopologues of Xe–SO3 have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. The complex is a symmetric top with a Xe–S van der Waals distance of 3.577(2) Å. The increase in rare gas distance relative to that in Kr–SO3 is equal to the difference in van der Waals radii between Xe and Kr. The 131Xe nuclear quadrupole coupling constant indicates that the electric field gradient at the xenon nucleus is 78% larger than that at the Kr nucleus in Kr–SO3.

The shape of D-glucosamine.

The bioactive amino monosaccharide D-glucosamine has been generated in the gas phase via laser ablation of D-glucosamine hydrochloride. Three cyclic α-(4)C1 pyranose forms have been identified using Fourier transform microwave techniques. Stereoelectronic hyperconjugative forces - essentially linked with the anomeric or gauche effect - and cooperative OH···O, OH···N and NH···O chains, extended along the entire molecule, are found to be the main factors driving the conformational behavior. The orientation of the NH2 group within each conformer has been determined by the values of the nuclear quadrupole coupling constants. The results have been compared with those recently obtained for the archetypical D-glucose.

Generation and structural characterization of aluminum cyanoacetylide.

Combined spectroscopy measurements and theoretical calculations bring to light a first investigation of a metallic cyanoacetylide, AlC3N, using laser ablation molecular beam Fourier transform microwave spectroscopy. This molecule was synthesized in a supersonic expansion by the reaction of aluminum vapour with C3N, produced from solid aluminum rods and BrCCCN in a newly constructed ablation-heating nozzle device. A set of accurate rotational and (27)Al and (14)N nuclear quadrupole coupling constants have been determined from the analysis of the rotational spectrum and compared with those predicted in a high-level ab initio study, conducting to the assignment of the observed species to linear AlCCCN. We have searched for this species towards the carbon-rich evolved star IRC + 10216 but only an upper limit to its abundance has been obtained.

Analytic first derivatives for a spin-adapted open-shell coupled cluster theory: evaluation of first-order electrical properties.

An analytic scheme is presented for the evaluation of first derivatives of the energy for a unitary group based spin-adapted coupled cluster (CC) theory, namely, the combinatoric open-shell CC (COSCC) approach within the singles and doubles approximation. The widely used Lagrange multiplier approach is employed for the derivation of an analytical expression for the first derivative of the energy, which in combination with the well-established density-matrix formulation, is used for the computation of first-order electrical properties. Derivations of the spin-adapted lambda equations for determining the Lagrange multipliers and the expressions for the spin-free effective density matrices for the COSCC approach are presented. Orbital-relaxation effects due to the electric-field perturbation are treated via the Z-vector technique. We present calculations of the dipole moments for a number of doublet radicals in their ground states using restricted open-shell Hartree-Fock (ROHF) and quasi-restricted HF (QRHF) orbitals in order to demonstrate the applicability of our analytic scheme for computing energy derivatives. We also report calculations of the chlorine electric-field gradients and nuclear quadrupole-coupling constants for the CCl, CH2Cl, ClO2, and SiCl radicals.

High-resolution spectroscopy of the chiral metal complex [CpRe(CH₃)(CO)(NO)]: a potential candidate for probing parity violation.

Heavy-metal containing chiral compounds have been suggested as promising candidates for studying parity-violation effects. We report herein the broadband rotational spectroscopy study of the chiral complex [CpRe(CH3)(CO)(NO)] in the gas phase. The spectra obtained are very rich due to the two rhenium isotopologues ((185)Re and (187)Re), hyperfine structure arising from the rhenium and nitrogen nuclei, and the asymmetry of the chiral complex. Since rhenium is located very close to the molecular center of mass, the rotational constants for the two rhenium isotopologues are very similar. However they can be differentiated by their characteristic nuclear quadrupole hyperfine splitting patterns. Comparison with calculated nuclear quadrupole coupling constants shows that all-electron relativistic basis sets are necessary for a correct description of the rhenium atom in this type of complex. The present study is an important step towards future precision studies on chiral molecules.

“EASY: A simple tool for simultaneously removing background, deadtime and acoustic ringing in quantitative NMR spectroscopy. Part II: Improved ringing suppression, application to quadrupolar nuclei, cross polarisation and 2D NMR”

A simple experiment for Elimination of Artifacts in NMR SpectroscopY (EASY) was introduced in Part I, and it was shown that NMR probe background signals, spectral distortions due to deadtime effects, and acoustic ringing can be eliminated simultaneously from solid-state NMR spectra. In this Part II, it is shown that acoustic ringing suppression can be improved up to one order of magnitude compared to the original EASY pulse sequence by inserting a delay τ between the two data acquisition scans of the EASY pulse sequence. The achievable ringing suppression depends on the length of this delay and is limited by the spin-lattice relaxation time T1. Furthermore, EASY is considered in conjunction with NMR of quadrupolar nuclei. For strong second-order broadening, EASY can be used to acquire either pure central transition MAS patterns or pure satellite transition NMR spectra. Two further modifications to EASY are introduced. One concerns improved ringing artifact suppression in experiments in which the central transition NMR signal is amplified by Rotor Assisted Population Transfer (RAPT). The second EASY modification enables the acquisition of quantitative NMR spectra if signals with different quadrupole coupling constants are present. In addition, acoustic ringing and 11B stator signals are removed. Finally, it is demonstrated that the basic idea of EASY for removing ringing artifacts can be realized for heteronuclear one-dimensional and hetero- and homo-nuclear multi-dimensional NMR experiments using extended phase cycling. 15N{1H} CPMAS and 15N 2D Exchange NMR spectroscopy are considered as examples.

Vibrational branching ratios and hyperfine structure of 11BH and its suitability for laser cooling

The simple structure of the BH molecule makes it an excellent candidate for direct laser cooling. We measure the branching ratios for the decay of the A^{1}Pi (v'=0) state to vibrational levels of the ground state, X^{1}Sigma^{+}, and find that they are exceedingly favourable for laser cooling. We verify that the branching ratio for the spin-forbidden transition to the intermediate a^{3}Pi state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that, with a relatively simple laser cooling scheme, a Zeeman slower and magneto-optical trap can be used to cool, slow and trap BH molecules.

Nitrogen split interstitial center(N−N)Nin GaN: High frequency EPR and ENDOR study

The nitrogen split interstitial defect introduced by high-energy particle irradiation in n-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and Q-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the g-tensor anisotropy, which is not resolved at X or Q band. The good agreement of the principal values g(xx) = 1.9966, g(yy) = 2.0016, and g(zz) = 2.0036 with the theoretically predicted g tensor confirm the (N-N)(N)(0) defect model. The hyperfine interactions of this defect have been studied byQ-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20-90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon interstitials is also the split interstitial configuration.

Structure ofLa130at low and medium spins

Low- and medium-spin states of the odd-odd nucleus $^{130}\mathrm{La}$ were studied by in-beam and pulsed-beam $\ensuremath{\gamma}$-ray spectroscopy using the $^{121}\mathrm{Sb}(^{12}\mathrm{C},3n)$ and $^{116}\mathrm{Sn}(^{16}\mathrm{O},pn)$ fusion-evaporation reactions. The decay out of the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ and $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{7/2}$ rotational structures to the ground state has been elucidated. Spin and parity values have been assigned to most of the newly observed states, as well as to the bandheads of the rotational bands. Two isomeric states were identified and placed in the level scheme, with the characteristics ${J}^{\ensuremath{\pi}}={5}^{+},{E}_{x}=214.0$ keV, ${T}_{1/2}=0.76(9)$ $\ensuremath{\mu}$s and ${J}^{\ensuremath{\pi}}={6}^{+},{E}_{x}=319.1$ keV, ${T}_{1/2}=33(1)$ ns. The magnetic and quadrupole interactions of the ${6}^{+}$ isomeric state have been investigated by the time-differential perturbed $\ensuremath{\gamma}$-ray angular distribution method. The $g$ factor value, $g({6}^{+})=+0.48(3)$, and the quadrupole coupling constant in polycrystalline metallic Sn lattice, ${\ensuremath{\nu}}_{Q}({6}^{+})=37(4)$ MHz, have been derived. The experimental data are compared with the predictions of two-quasiparticles plus rotor model calculations.

Dynamics of methanol in ionic liquids: validity of the Stokes-Einstein and Stokes-Einstein-Debye relations.

The validity of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations for methanol in the physical environment of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is studied by means of nuclear magnetic resonance (NMR) relaxation time experiments, viscosity measurements and molecular dynamics (MD) simulations. The reorientational correlation times of the hydroxyl groups of pure methanol and of methanol in the IL/methanol mixtures were determined. For that purpose an approach for estimating NMR deuteron quadrupole coupling constants, presented by Wendt and Farrar (Mol. Phys. 1998, 95, 1077-1081), was confirmed. The self-diffusion coefficients of methanol were taken from the MD simulations. The viscosities of all systems were then measured and the SE and SED relations validated. For pure methanol both relations are valid, whereas they become increasingly invalid with increasing IL concentration, as indicated by effective volumes and radii that are too low. The deviation from the SE and SED relations could be related to dynamical heterogeneities described by the non-Gaussian parameter α(t) obtained from MD simulations. For pure methanol, α(t) is close to zero in accord with the validity of both relations. With increasing IL concentration the dynamical heterogeneities of methanol increase strongly. The times t* at the maximum of α(t) increase linearly with the relative number of methanol monomers in the mixtures. Thus, the dynamical heterogeneities are largest for single methanol molecules fully embedded in the IL environment. In their own environment methanol molecules are highly mobile, whereas in the IL-rich region the mobility is strongly reduced leading to the non-validity of SE and SED relations.

Nuclear quadrupole coupling constants of two chemically distinct nitrogen atoms in 4-aminobenzonitrile.

The rotational spectrum of 4-aminobenzonitrile in the gas phase between 2 and 8.5 GHz is reported. Due to the two chemically distinct nitrogen atoms, the observed transitions showed a rich hyperfine structure. From the determination of the nuclear quadrupole coupling constants, information about the electronic environment of these atoms could be inferred. The results are compared to data for related molecules, especially with respect to the absence of dual fluorescence in 4-aminobenzonitrile. In addition, the two-photon ionization spectrum of this molecule was recorded using a time-of-flight mass spectrometer integrated into the setup. This new experimental apparatus is presented here for the first time.

Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View

The nature of γ-alumina (γ-Al2O3) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated 27Al NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of γ-Al2O3. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (CQ) values (20–34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic “defect sites”, and gradual lowering to bulklike CQ values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local...

Identification of the Zn Substitution Sites in La–Zn Substituted SrAl12O19 from 27Al Solid‐State NMR Studies

The La–Zn substituted hexagonal strontium aluminate, Sr1−xLaxAl12−xZnxO19, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using 27Al solid‐state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr1−xLaxFe12−xCoxO19, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f2 AlO6 octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta‐coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr1−xLaxFe12−xCoxO19, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.

Feasibility of arsenic and antimony NMR spectroscopy in solids: An investigation of some group 15 compounds

The feasibility of obtaining 75As and 121/123Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, 121/123Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide “salts” are reflected in the measured 75As and 121Sb nuclear quadrupole coupling constants, CQ(75As)=7.8MHz and CQ(121Sb)=159MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of 75As and 121Sb NMR spectra (at 154 and 215MHz, respectively, i.e., at B0=21.14T) with linewidths in excess of 1MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).

Terahertz spectroscopy of methanimine and its isotopologs

Context. Methanimine (CH2NH) is a simple molecule composed of methylene and imine. The molecule has been detected toward the Galactic center, star-forming regions, circumstellar envelopes, and other galaxies since 1973. In previous studies, the rest frequency of methanimine has been measured for normal species up to the 650-GHz region, but its 13 CH2NH, CH2 15 NH, and CH2ND isotopologs were limited to the 100-GHz region. Aims. If a rotational temperature of 100 K is assumed for methanimine, the highest intensity falls at approximately 1.5 THz. In addition to normal methanimine, the 13 CH2NH, CH2 15 NH, and CH2ND isotopologs in their ground-vibrational states were observed in the frequency range of 120–1600 GHz to provide accurate rest frequency information. Based on this study, the calculated rest frequencies below 2 THz should be sufficiently precise and support observations using all ALMA and Herschel/HIFI observational bands. Methods. Methanimine was generated by pyrolysis of diaminoethane (DAE) vapor at 850 ◦ C. 13 CH2NH and CH2 15 NH isotopologs were measured with their natural abundance, and deuterization of DAE was performed by mixing normal DAE with deuterated water, D2O, and then pyrolyzed. This gives the deuterated isotopolog of methanimine, CH2ND. Spectral measurements were performed by using the 23 kHz source-frequency modulated terahertz spectrometer at Toho University. Results. Both a -a ndb-type transitions up to 1.6 THz for the three isotopologs and the normal species were measured. Rotational and centrifugal distortion constants for the three isotopologs were accurately determined. For normal species (CH2NH), both electric quadrupole and nuclear spin-rotation coupling constants for nitrogen nucleus were determined, while for the 13 CH2NH and CH2ND species, only electric quadrupole-coupling constants for nitrogen nucleus were determined. Conclusions. Our spectral line frequencies are suitable for a future astronomical search for these isotopologs of methanimine. The 1σ frequency accuracy up to 2 THz is lower than 100 kHz.