Ground-state Rotational Spectra Research Articles

Rotational spectroscopy and structure of 1,1-dichloro-1-silacyclohex-2-ene

The ground state rotational spectrum of 1,1-dichloro-1-silacyclohex-2-ene has been recorded using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. Several isotopologues in their natural abundances have been observed in the free-jet expansion, and their spectra assigned, making it possible to present a partial heavy-atom substitution structure. Furthermore, the high resolution of this technique allows the complicated hyperfine splitting pattern to be largely deconvoluted. As a result, the on-diagonal nuclear quadrupole coupling constants for the two chlorine atoms have been determined for all observed isotopologues. Additionally, χbc is determined for both chlorine atoms of the parent species. The quadrupole coupling tensors for the parent species have been diagonalised, noting some assumptions have been made pertaining to the off-diagonal nuclear quadrupole coupling constants in the principal axis system, to yield reasonable values of χzz and η which are then compared.

Millimeter to THz Spectroscopy of HC18O+ and HC17O+: Accurate Rest Frequencies for Astrophysical Studies

Heavy oxygen isotopic species of HCO+ are important optically thin astrophysical tracers. The ground-state rotational spectrum of HC18O+, DC18O+, HC17O+, and DC17O+ has been recorded in the laboratory in the frequency range from 85 GHz to 1.3 THz. The ions have been produced in the negative column of a glow-discharge plasma, and their spectrum has been recorded in absorption using a frequency-modulation submillimeter-wave spectrometer. Various sources of systematic error have been carefully accounted for in order to obtain highly accurate line-position measurements. Theoretical estimates of the molecular parameters and of the collision effects on the line shape have been obtained by high-level ab initio calculations. The analysis yielded much improved rotational and centrifugal distortion constants, thus bringing the spectroscopic characterization of these rare isotopic variants to the same level of the parent species. Also, the first experimental rotational data for DC17O+ have been provided. These results allow for the calculation of an updated set of rest frequencies to support current and future astrophysical studies. The derived data set for the widely used HC18O+ tracer reaches an accuracy of a few parts in 109 up to 1.5 THz. Such accuracy is important for the analysis of astrophysical objects targeted by Atacama Large Millimeter/submillimeter Array observations at the submillimeter regime.

Structure and nuclear quadrupole coupling interaction in hydroxylamines: The rotational spectrum of N,N-diethyl(2H)hydroxylamine

The ground-state rotational spectrum of N,N-diethyl(2H)hydroxylamine (C2H5-NOD-C2H5) was measured by Fourier transform microwave spectroscopy. Six rotational transition lines were assigned to the most stable conformer with the alkyl chain in the trans arrangement and the hydroxyl trans to the bisector of the CNC angle, that is, with the NOD frame in the bc-symmetry plane. Rotational constants are A= 7210.938(2), B= 2018.628(1), and C= 1739.696(1) MHz. These data together with those previously determined for the parent species and 13C and 15N isotopologues, were used to determine a partial r0 structure. The hyperfine structure due to the nuclear quadrupole coupling (NQC) interaction of both 14N and D nuclei was disentangled allowing to obtain the diagonal NQC-constants. On the basis of the determined geometry, the NQC-tensor values in the electric field-gradient principal axis system were determined (χxxN= 0.71, χyyN= 5.90, χzzN= −6.61 MHz; χxxD= −0.11, χyyD= −0.15, χzzD= 0.26 MHz). Comparison with other amines shows that using ammonia as reference, χzzN can be estimated with an additive model: +0.4 and +1.7 MHz from hydrogen to alkyl and hydroxyl substitution, respectively. From the analysis of the available data on the 1:1 water complex of N,N-diethylhydroxylamine, a 8% electric field gradient decrease at the nitrogen nucleus due to the formation of the hydrogen bond involving the nitrogen lone pair was found.

Rotational spectrum of Isochroman

The ground state rotational spectrum of isochroman (3,4-dihydro-1H-2-benzopyran, C9H10O) has been studied by using a pulsed jet Fourier transform microwave spectrometer in the range of 5-20 GHz, complemented with theoretical calculations. Accurate spectroscopic parameters including rotational and centrifugal distortion constants were determined for the parent species and all mono-substituted 13C isotopologues in natural abundance, which allow the determination of the accurate geometry of the carbon skeleton. The semi-experimental structure was evaluated by taking into account the vibrational corrections calculated from the B3LYP-D3(BJ)/6-311++G(d,p) anharmonic force field. The electrostatic potential surface above the benzene ring with different saturated cycles (n = 3-7) has been calculated at the MP2/6-311++G(d,p) level, implying that the electron withdrawing effect of electronegative O and N atoms can be transferred from the adjacent saturated ring to the benzene ring.

Accuracy of the equilibrium structure of sulphur dioxide

The ground state rotational spectra of 12 different isotopologues of sulphur dioxide have been refitted in a consistent way and an accurate experimental cubic force field has been determined. The experimental, semi-experimental and ab initio equilibrium structures have been calculated and found in good agreement across themselves. Different approximations limiting the accuracy are discussed. In particular, the influence of centrifugal distortion is analysed. A method is proposed to estimate the higher-order rovibrational corrections (γ-constants). Although an accurate semi-experimental structure was obtained for 19 isotopologues, it was not possible to determine and account for the breakdown of the Born–Oppenheimer approximation. Finally, it was shown that the ab initio CCSDTQ level of theory was required to obtain a good agreement with the experimental structure.

A prochiral precursor in space? Accurate laboratory characterization of acetylacetylene in the cm-wave region

The most prominent pure rotational transitions of the prochiral, possibly prebiotic molecule acetylacetylene are recorded in the frequency range of 6.5–26.5 GHz for a reliable spectral characterization prerequisite to a possible extraterrestrial observation. Therefore, three different approaches for global fitting of the vibrational ground state are applied to analyze the experimentally determined transition frequencies using the programs SPFIT, XIAM and ERHAM corresponding to different theoretical treatments solving the internal rotation problem.Furthermore, the resulting internal rotation parameter V3 is compared to those other substituted acetyl species, and derivatives of butynes and possible explanations for the observed trends are given. For the substituted acetyl species, the spectra of acetylfluoride, -chloride, -bromide and -iodide are reanalyzed.The obtained rotational constants have been interpreted in terms of characterizing the geometric structure. A correlation between the tunneling parameter ε1 and V3 of several molecular species displaying spectral internal rotation splitting patterns has been found and quantified. This relationship can simplify the analysis of vibrational ground state rotational spectra in finding a suitable starting point for the fitting procedure using the program ERHAM, which is the predestined choice to analyze and predict signal positions up to very high quantum numbers. Being able to predict those signal positions is crucial for a possible comprehensive astronomical identification and subsequent application as an astrophysical probe for the conditions in interstellar molecular clouds.

Derivation of an accurate geometry of 2-fluoroaniline from rotational spectroscopy and computational chemistry

The ground state rotational spectrum of 2-fluoroaniline was investigated using Fourier transform microwave spectroscopy in the 6-19 GHz range. The spectrum reveals hyperfine structure due to the 14N quadrupole moment and additional transitions due to the six 13C and one 15N minor isotopologues which were detected in natural abundance. The rotational constants derived from analysis of the observed spectral patterns for the eight isotopologues were used to derive accurate experimental geometries including the substitution (rs), ground state effective (r0) and mass dependence (rm(1)) structures of 2-fluoroaniline. These show excellent agreement with the equilibrium (re) geometry derived from quantum chemical calculations at the B3LYP/aug-cc-pVTZ level of theory. The geometry of the heavy atom backbone of 2-fluoroaniline, in comparison to the parent aniline, is consistent with effects from the electron withdrawing fluorine atom and from the presence of a weak non-covalent interaction between the neighbouring NH2 and F groups as supported by natural bond orbital and non-covalent interaction analyses, respectively.

Bifunctional Hydrogen Bonding of Imidazole with Water Explored by Rotational Spectroscopy and DFT Calculations

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H2O and H2O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H2O, but these roles are reversed in the H2O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H2O and three isotopologues of H2O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A0, B0, C0), centrifugal distortion (DJ, DJK), and nuclear quadrupole coupling constants (χaa(N1), [χbb(N1) - χcc(N1)], χaa(N3), and [χbb(N3) - χcc(N3)]). Structural parameters (r0 and rs) have been accurately determined from measured rotational constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O-Hb···N3) = 172.1(26)° in the experimentally determined, r0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H2O. The DFT calculations find that the H2O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1-N1) = 174.7°). Two states observed in the spectrum of H2O···imid, assigned as 0- and 0+ states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.

Rotational spectra of 2,3,6-trifluoropyridine: Effect of fluorination on ring geometry

The ground state rotational spectrum of 2,3,6-trifluoropyridine has been investigated in the 2.0–20.0 GHz region by pulsed jet Fourier transform microwave spectroscopy. The experimental rotational constants are A=3134.4479(2) MHz, B=1346.79372(7) MHz, and C=941.99495(6) MHz. The transitions are so intense that rotational transitions of all mono-13C and 15N isotopologues are measured in natural abundance. The semi-experimental equilibrium rotational constants of the 7 isotopologues were derived by taking account of the anharmonic vibrational corrections, which allowed a semi-experimental determination of the equilibrium structure of 2,3,6-trifluoropyridine.

A chalcogen-bonded complex H3N⋯S=C=S formed by ammonia and carbon disulfide characterised by chirped-pulse, broadband microwave spectroscopy.

Ground-state rotational spectra were observed for ten symmetric-top isotopologues H3N⋯S=C=S, H3N⋯34S=C=S, H3N⋯S=C=34S, H3N⋯S=13C=S, H3 15N⋯S=C=S, H3 15N⋯34S=C=S, H3 15N⋯S=C=34S, H3 15N⋯S=13C=S, H3 15N⋯33S=C=S, and H3 15N⋯S=C=33S, the first five in their natural abundance in a mixture of ammonia and carbon disulphide in argon and the second group with enriched 15NH3. The four asymmetric-rotor isotopomers H2DN⋯S=C=S, H2DN⋯34S=C=S, H2DN⋯S=C=34S, and HD2N⋯S=C=S were investigated by using a sample composed of ND3 mixed with CS2. Rotational constants, centrifugal distortion constants, and 33S nuclear quadrupole coupling constants were determined from spectral analyses and were interpreted with the aid of models of the complex to determine its symmetry, geometry, one measure of the strength of the intermolecular binding, and information about the subunit dynamics. The complex has C3v symmetry, with nuclei in the order H3N⋯S=C=S, thereby establishing that the non-covalent interaction is a chalcogen bond involving the non-bonding electron pair of ammonia as the nucleophile and the axial region near one of the S atoms as the electrophile. The small intermolecular stretching force constant kσ = 3.95(5) N m-1 indicates a weak interaction and suggests the assumption of unperturbed component geometries on complex formation. A simple model used to account for the contribution of the subunit angular oscillations to the zero-point motion leads to the intermolecular bond length r(N⋯S) = 3.338(10) Å.

Accurate sub-millimetre rest frequencies for HOCO+ and DOCO+ ions

HOCO$^+$ is a polar molecule that represents a useful proxy for its parent molecule CO$_2$, which is not directly observable in the cold interstellar medium. This cation has been detected towards several lines of sight, including massive star forming regions, protostars, and cold cores. Despite the obvious astrochemical relevance, protonated CO$_2$ and its deuterated variant, DOCO$^+$, still lack an accurate spectroscopic characterisation. The aim of this work is to extend the study of the ground-state pure rotational spectra of HOCO$^+$ and DOCO$^+$ well into the sub-millimetre region. Ground-state transitions have been recorded in the laboratory using a frequency-modulation absorption spectrometer equipped with a free-space glow-discharge cell. The ions were produced in a low-density, magnetically-confined plasma generated in a suitable gas mixture. The ground-state spectra of HOCO$^+$ and DOCO$^+$ have been investigated in the 213-967 GHz frequency range, with the detection of 94 new rotational transitions. Additionally, 46 line positions taken from the literature have been accurately remeasured. The newly-measured lines have significantly enlarged the available data sets for HOCO$^+$ and DOCO$^+$, thus enabling the determination of highly accurate rotational and centrifugal distortion parameters. Our analysis showed that all HOCO$^+$ lines with Ka $\geq$ 3 are perturbed by a ro-vibrational interaction that couples the ground state with the v$_ 5$ = 1 vibrationally-excited state. This resonance has been explicitly treated in the analysis in order to obtain molecular constants with clear physical meaning. The improved sets of spectroscopic parameters provide enhanced lists of very accurate, sub-millimetre rest-frequencies of HOCO$^+$ and DOCO$^+$ for astrophysical applications. These new data challenges a recent tentative identification of DOCO$^+$ toward a pre-stellar core.

Fourier Transform Microwave Spectrum of Propene-3-d1 (CH2═CHCH2D), Quadrupole Coupling Constants of Deuterium, and a Semiexperimental Equilibrium Structure of Propene.

The ground-state rotational spectrum of propene-3-d1, CH2═CHCH2D, was measured by Fourier transform microwave spectroscopy. Transitions were assigned for the two conformers, one with the D atom in the symmetry plane (S) and the other with the D atom out of the plane (A). The energy difference between the two conformers was calculated to be 6.5 cm-1, the S conformer having lower energy. The quadrupole hyperfine structure due to deuterium was resolved and analyzed for both conformers. The experimental quadrupole coupling and the centrifugal distortion constants compared favorably to their ab initio counterparts. Ground-state rotational constants for the S conformer are 40582.157(9), 9067.024(1), and 7766.0165(12) MHz. Ground-state rotational constants for the A conformer are 43403.75(3), 8658.961(2), and 7718.247(2) MHz. For the A conformer, a small tunneling splitting (19 MHz) due to internal rotation was observed and analyzed. Using the new rotational constants of this work as well as those previously determined for the 13C species and for some deuterium-substituted species from the literature, a new semiexperimental equilibrium structure was determined and its high accuracy was confirmed. The difficulty in obtaining accurate coordinates for the out-of-plane hydrogen atom is discussed.

Three isomers detected for the whisky lactone: 5-butyl-4-methyl tetrahydrofuran-2-one by Fourier transform microwave spectroscopy and quantum chemical calculations

The ground-state rotational spectra of the whisky lactone (WL): 5-butyl-4-methyl tetrahydrofuran-2-one were observed and analyzed by molecular beam Fourier transform microwave spectroscopy combined with quantum chemical calculations. We have detected three stereo-isomers: the trans-TTT form with a methyl CH3 group attached to C(4) in an equatorial position (eq) and a butyl C4H9 group to C(5) in an eq position, for which 110 b-type and 113 a-type transitions were assigned, the cis-TTT form with a CH3 to C(4) in an axial position (ax) and a C4H9 to C(5) in eq, for which 96 a-type, 101 b-type, and 45 c-type transitions were observed, and the cis-GTT form with a CH3 to C(4) in ax and a C4H9 to C(5) in eq, for which 158 a-type, 52 b-type, and 17 c-type transitions were observed, where TTT and GTT denote the conformations about the C(6)C(5), C(7)C(6), and C(8)C(7) bonds, with T and G designating trans and gauche, respectively. The rotational constants thus derived agree with the predictions made by quantum chemical calculations, MP2/6-311++G(d, p) within 1.2%. The trans-TTT form was calculated to be the most stable. The splittings due to internal rotation of the terminal methyl in the butyl group were observed for all the three stereo-isomers and were analyzed by the XIAM program to determine the threefold potential barrier V3 to be 966.4 (25), 978.8 (11), and 1098.7 (48) in cm−1 for the trans-TTT (eq, eq), the cis-TTT (ax, eq), and the cis-GTT (ax, eq) forms, respectively, to be compared with quantum chemically calculated values: 1055, 1055, and 1053 in cm−1.

A complex Ar⋯Ag[formula omitted]I produced by laser ablation and characterised by rotational spectroscopy and ab initio calculations: Variation of properties along the series Ar⋯Ag[formula omitted]X (X = F, Cl, Br and I)

A complex of argon with silver iodide (Ar⋯AgI) has been formed in the gas phase by laser ablation of a silver iodide rod in the presence of a pulse of argon gas and its ground-state rotational spectrum has been detected by means of a chirped-pulse, F-T microwave instrument. Ar⋯AgI was characterised both by experimental properties determined from its rotational spectrum and by ab initio calculations carried out at the CCSD(T)(F12c)/cc-pVTZ-F12 explicitly correlated level of theory. The molecule was shown to be linear in the ground state, with atoms in the order shown. The Ar⋯Ag and AgI bond lengths r0(Ar⋯Ag)=2.6759Å and r0(AgI)=2.5356Å, the dissociation energy De=16.7kJmol−1 for the process Ar⋯AgI=Ar+AgI, the intermolecular quadratic stretching force constant FAr⋯Ag=F22=20.2(8)Nm−1 and the increase 0.033 in the ionicity ic of AgI when it enters the complex are reported. The opportunity has been taken to compare the way in which these properties vary along the series Ar⋯AgX (X=F, Cl, Br and I).

Dimethyl Sulfide-Dimethyl Ether and Ethylene Oxide-Ethylene Sulfide Complexes Investigated by Fourier Transform Microwave Spectroscopy and Ab Initio Calculation.

The ground-state rotational spectra of the dimethyl sulfide-dimethyl ether (DMS-DME) and the ethylene oxide-ethylene sulfide (EO-ES) complexes were observed by Fourier transform microwave spectroscopy, and a-type and c-type transitions were assigned for the normal, (34)S, and three (13)C species of the DMS-DME and a-type and b-type transitions for the normal, (34)S, and two (13)C species of the EO-ES complexes. The transition frequencies measured for both the complexes were analyzed by using an S-reduced asymmetric-top rotational Hamiltonian. The rotational parameters thus derived for the DMS-DME were found to be consistent with a structure of Cs symmetry with the DMS bound to the DME by two C-H(DMS)···O and one S···H-C(DME) hydrogen bonds. Some high-Ka lines were found to be split, and we have interpreted these splittings in terms of internal rotations of the two methyl groups of the DMS and of the "free", i.e., outer group, of the DME. Some forbidden transitions were also observed in cases where Ka = 3 levels were involved, for the DMS-DME complex in the internal-rotation E state. The barrier height, V3, to internal rotation of the CH3 in the DME thus derived is smaller than that of the DME monomer, while the V3 of the CH3 groups in the DMS is nearly the same as that of the DMS monomer. For the EO-ES complex, the observed data were interpreted in terms of an antiparallel structure of Cs symmetry with the EO bound to the ES by two C-H(ES)···O and two S···H-C(EO) hydrogen bonds. An attempt was also made to observe a-type transitions of the DMS dimer without success. We have applied a natural bond orbital analysis to the DMS-DME and EO-ES to calculate the stabilization energy CT (= ΔEσσ*), which was correlated closely with the binding energy as found for other related complexes.

Investigation of Structural Trends in Mono-, Di-, and Pentafluorobenzonitriles Using Fourier Transform Microwave Spectroscopy

The ground state rotational spectra of a series of fluorinated benzonitriles (BN), namely, 2-fluorobenzonitrile (2FBN), 3-fluorobenzonitrile (3FBN), 2,3-difluorobenzonitrile (23DFBN), 2,4-difluorobenzonitrile (24DFBN), and pentafluorobenzonitrile (PFBN), have been investigated between 4 and 24 GHz using Fourier transform microwave (FTMW) spectroscopy. The assigned transitions include those due to the parent as well as the (13)C and (15)N singly substituted isotopologues which were observed in natural abundance. The spectroscopic analysis allowed the derivation of substitution (rs) and effective ground state structures (r0) to investigate the effect of mono-, di-, and pentafluoro substitution on the geometry of the BN backbone and are compared here with ab initio values of the equilibrium parameters (re) obtained from MP2/6-311++G(2d,2p) calculations. Analysis of the (14)N hyperfine structure provides additional information about the electronic structure surrounding the nitrogen atom. The observed geometry changes relative to the reference BN compound are interpreted using natural bond orbital (NBO) analysis to describe differences in the hybridization at various sites and contributions from plausible resonance structures.

Intermolecular interaction in the formaldehyde-dimethyl ether and formaldehyde-dimethyl sulfide complexes investigated by Fourier transform microwave spectroscopy and ab initio calculations.

The ground-state rotational spectra of the formaldehyde-dimethyl ether (H2CO-DME) and formaldehyde-dimethyl sulfide (H2CO-DMS) complexes have been studied by Fourier transform microwave spectroscopy. The a-type and c-type rotational transitions have been assigned for the normal and deutrated formaldehyde-containing species of both complexes. In the case of H2CO-DME, doublets were observed with the splitting 10-300 kHz, whereas no such splittings were observed for H2CO-DMS, D2CO-DME, and D2CO-DMS. The observed rotational spectra were found consistent with a structure of Cs symmetry with DME or DMS bound to H2CO by two types of hydrogen bonds: C-H(DME/DMS)---O(H2CO) and O(DME)/S(DMS)---H-C(H2CO). The R(cm) distances between the centers of mass of the component molecules in the H2CO-DME and H2CO-DMS complexes were determined to be 3.102 and 3.200 Å, respectively, which are shorter than those in most related complexes. The spectral and NBO analyses showed that H2CO-DMS has a stronger charge transfer interaction than H2CO-DME does and that the binding energy of H2CO-DMS is larger than that of H2CO-DME.

Distortions of ethyne when complexed with a cuprous or argentous halide: the rotational spectrum of C2H2···CuF.

A new molecule C2H2···CuF has been synthesized in the gas phase by means of the reaction of laser-ablated metallic copper with a pulse of gas consisting of a dilute mixture of ethyne and sulfur hexafluoride in argon. The ground-state rotational spectrum was detected by two types of Fourier-transform microwave spectroscopy, namely that conducted in a microwave Fabry-Perot cavity and the chirped-pulse broadband technique. The spectroscopic constants of the six isotopologues 12C2H2···63Cu19F, 12C2H2···65Cu19F, 13C2H2···63Cu19F, 13C2H2···65Cu19F, 12C2D2···63Cu19F and 12C2D2···65Cu19F were determined and interpreted to show that the molecule has a planar, T-shaped geometry belonging to the molecular point group C 2v, with CuF forming the stem of the T. Quantitative interpretation reveals that the ethyne molecule is distorted when subsumed into the complex in such manner that the C[triple bond, length as m-dash]C bond lengthens (by δr) and the two H atoms cease to be collinear with the C[triple bond, length as m-dash]C internuclear line. The H atoms move symmetrically away from the approaching Cu atom of CuF, to increase each *[triple bond, length as m-dash]C-H angle by δA = 14.65(2)°, from 180° to 194.65(2)°. Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12*)/aug-cc-pVTZ lead to good agreement with the experimental geometry. It is shown that similar distortions δr and δA, similarly determined, for four complexes C2H2···MX (M = Cu or Ag; X = F, Cl or CCH) are approximately linearly related to the energies D e for the dissociation process C2H2···MX = C2H2 + MX.

Microwave Spectroscopic Investigation and Structural Determination of the Ar–Difluoropyridine van der Waals Complexes

The ground state rotational spectra of the van der Waals dimers of 2,3-, 2,4-, 2,5-, 2,6-, and 3,5-difluoropyridine with argon were investigated in the range of 4 to 26 GHz using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopy. The spectroscopic parameters derived from fitting the observed transitions were used to estimate the van der Waals coordinates of the five weakly bound complexes and to quantitatively describe their internal dynamics using a pseudodiatomic model. The experimental structures are in fair agreement with MP2 calculations (6-311++G(2d, 2p)) and have the centers of mass of the two moieties separated by 3.486 to 3.545 Å with the Ar atom lying 2-10° from the c' axis of the ring monomer.

Rotational spectroscopy of methylamine up to 2.6 THz

Context. Methylamine (CH3NH2) is the simplest primary alkylamine that has been detected in the interstellar medium. The molecule is relatively light, with the 50 K Boltzmann peak appearing near 800 GHz. However, reliable predictions for its rotational spectrum are available only up to 500 GHz. Spectroscopic analyses have been complicated by the two large-amplitude motions: internal rotation of the methyl top and inversion of the amino group. Aims. To provide reliable predictions of the methylamine ground state rotational spectrum above 500 GHz, we studied its rotational spectrum in the frequency range from 500 to 2650 GHz. Methods. The spectra of methylamine were recorded using the spectrometers based on Schottky diode frequency multiplication chains in the Lille laboratory (500−945 GHz) and in JPL (1060−2660 GHz). The analysis of the rotational spectrum of methylamine in the ground vibrational state was performed on the basis of the group-theoretical high barrier tunneling Hamiltonian developed for methylamine by Ohashi and Hougen. Results. In the recorded spectra, we have assigned 1849 new rotational transitions of methylamine. They were fitted together with previously published data, to a Hamiltonian model that uses 76 parameters with an overall weighted rms deviation of 0.87. On the basis of the new spectroscopic results, predictions of transition frequencies in the frequency range up to 3 THz with J ≤ 50 and Ka ≤ 20 are presented.