Theoretical Spectroscopic Study of Isopropyl Alcohol (CH3-CHOH-CH3)

Accurate spectroscopic parameters of isopropyl alcohol, a volatile organic compound present on Earth and in extraterrestrial atmospheres, are provided. The work pursues the study of the far-infrared region, describing the distribution of the low-lying vibrational energy levels that can be populated at very low temperatures, having effects on the detectability and identification. The potential energy surface shows 27 almost equivalent minima producing 27 subcomponents of the levels due to tunneling effects. Computations have been performed using highly correlated ab initio calculations, accurate enough to distinguish between the rotational parameters of the two quasi-identical conformers gauche and trans. A variational procedure in three dimensions depending on three interacting internal rotations, the internal rotation of the two methyl groups and the internal rotation of the hydroxyl group, is employed to compute the energies. The 27 subcomponents of the ground vibrational state can be grouped into three series of nine energy levels located around 0.000, 1.693, and 81.927 cm−1 whose energy difference is due to the OH torsion effect. The nine subcomponents integrated in each series derive from the torsion of the two methyl groups. The computations reproduce accurately the available experimental data. New predicted properties can help the spectroscopic analysis of the rotational-vibrational spectra in the gas phase and further detections of vibrationally excited isopropyl alcohol.


Introduction
The simplest secondary alcohol, isopropyl alcohol (2propanol, i-propanol, CH 3 CHOHCH 3 ), is an oxygenated volatile organic compound present in terrestrial and extraterrestrial atmospheres.It represents a nonrigid molecule where two different conformers separated by very low energy barriers inter-transform through internal rotation.This behavior, which is common for many complex biological species, contributes to spectral confusion and can have costs for molecular identification.Detection in gas phase sources through large-scale atmospheric or astrophysical instruments implies a previous laboratory characterization, which is devious due to the density of vibrational states of low energies.Theoretical procedures can be employed to understand the far-infrared region mapping the distribution of low-lying vibrational levels.
Very recently, i-C 3 H 7 OH has been detected in the high-mass star-forming protocluster Sagittarius Sgr B2(N2) with the Atacama Large Millimeter/submillimeter Array (Belloche et al. 2022).It has contributed to increasing the catalog of nonrigid organic species discovered in the gas phase in extraterrestrial sources.In SrgB2 (N), isopropanol was found to be nearly as abundant as its isomer n-propanol, with an abundance ratio of 0.6, which is similar to the ratio of 0.4 for iso-and normal-propyl cyanide in Sgr B2(N2) (Belloche et al. 2014;Garrod et al. 2017;Kerkeni et al. 2019Kerkeni et al. , 2023)).
Astrochemical models (Garrod et al. 2017) suggest that the OH radical addition to propylene in dust-grain ice mantles, driven by water photodissociation, can produce appropriate quantities of n-and i-propanol.
Furthermore, searches in the interstellar medium (ISM) of the two propanol isomers have been motivated by the possibility that alcohols can be precursors of lipids in the primitive Earth (Jimenez-Serra et al. 2022).However, while n-propanol (CH 3 CH 2 CH 2 OH) has been detected toward the Giant Molecular Cloud G+0.693-0.027 in the Galactic Center using the IRAM 30 m and Yebes 40 m telescopes, the two conformers of isopropanol were not observed (Jimenez-Serra et al. 2022).The abundance ratio with respect to methanol in this cloud is significantly different than the one found toward SrgB2 (N).The invisibility of molecules that have been postulated to be detectable species can be related to a lack of laboratory data.Structural, spectroscopic, or collisional properties (Senent et al. 2012), as well as reactivity, can justify disagreements in the determination of abundances affecting molecular identification.
At room temperature, isopropyl alcohol is a liquid commonly used as a solvent although it has many uses in the production of a wide variety of industrial products and household chemicals.The presence of alcohols in the Earth's atmosphere is due to both natural and anthropogenic sources (Derwent et al. 2010;Mellouki et al. 2015) and can contribute to the formation of tropospheric ozone through more or less complex networks of reactions where oxidative radicals can play an important role.In the Photochemical Ozone Creation Potentials scale (POPC; Derwent et al. 2010), isopropyl alcohol has a similar classification to methanol.The POPC represents a comparative tool for chemical mechanisms in the gas phase, Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
which allows us to compare reaction efficiencies relative to ethylene.The smaller alcohols are highly water soluble and highly volatile, properties that simplify their removal from the atmosphere.In the gas phase, aliphatic alcohols are oxidized through their reaction with OH and NO 3 radicals, with the OH radical reaction being dominant whereas reactions with ozone and photolysis are negligible processes.The oxidation of alcohols can produce carbonyl compounds, such as aldehydes and ketones.Indeed, the oxidation of isopropyl alcohol easily leads to the formation of acetone (Wiberg & Schaefer 1969).The reaction of isopropyl alcohol with hydrocarbon radicals has been studied in the gas phase (Herod 1968).
Complex nonrigid biological molecules are mainly detected in the ISM through radio-astronomy, requiring previous measurements and assignments of the rotational spectra.The astrophysical search has motivated recent microwave studies (Maeda et al. 2006), although the interest in isopropyl alcohol and its structural and spectroscopic properties started very early.Schumann & Aston (1938) published a work focused on the effect of the internal rotation on the thermochemical properties of acetone and isopropyl alcohol.In 1963, Green measured the infrared spectra and Raman spectra in the 4000-200 cm −1 region.The assigned frequencies were used together with molecular structure data to compute thermodynamic properties.Tanaka (1962) suggested the existence of the trans and gauche forms from the study of the middle infrared region in the gas phase, and Kondo & Hirota (1970) detected two rotational isomers, trans and gauche, for the isopropyl alcohol molecule from the analysis of the microwave spectrum.The a-type lines, characteristic of the gauche form, were observed.The barrier height was obtained to be V t = 1.68 kcal/mol(V t = 587.6 cm −1 ) by assuming a threefold symmetry for the internal rotation of the OH group.Later on, Inagaki et al. (1973) observed in the far-infrared spectrum two prominent Q branches at 209 and 234 cm −1 , which they assigned to the OH torsional fundamental of the trans and gauche conformers, respectively.
The microwave spectrum of isopropyl alcohol has been studied by Hirota (1979).For the gauche form, the microwave spectrum was investigated in the region 10-80 GHz by Ulenikov et al. (1991).A very significant contribution is the recent work of Maeda et al. (2006), who studied the millimeter-and submillimeter-wave region of the electromagnetic spectrum through 360 GHz.They successfully assigned ∼7600 pure rotational transitions within the torsional substates, as well as ∼4700 torsional-rotational transitions between the symmetric and antisymmetric gauche substates through the lower rotational quantum number J″ = 68.Spectra of deuterated varieties have been also measured and assigned (Hirota & Kawashimay 2001;Dobrowolski et al. 2008).Previous ab initio calculations were performed to determine stabilities and the energy barriers of internal rotation (Sun & Bozzelli 2002;Kirschner et al. 2018).
All these previous laboratory studies underline effects derived from the nonrigidity of isopropyl alcohol.Due to three interacting internal rotations (those of the two methyl groups and the hydroxyl group), the potential energy surface displays 27 quasi-equivalent minima separated by low energy barriers and corresponding to two different conformers gauche and trans.This causes 27 splitting components of each level and vibrational transitions involving gauche and gauche, trans and trans, and trans and gauche substates.A large density of lowenergy states is expected to produce complex spectral features in the far-infrared region.Effects of the nonrigidity can be observed with many different spectroscopic techniques through transitions connecting combination states involving torsional excitations and excitations of other vibrational modes.
In the meticulous work of Maeda et al. (2006), the gauche substates were analyzed using a 2 × 2 Hamiltonian and assuming that in its trans form, isopropanol is a simple semirigid asymmetric top.For the ground vibrational state, they obtain an energy difference between the gauche components of 3.62 ± 0.96 cm −1 clearly larger than the value of 1.561 cm −1 obtained from direct measurement using microwave spectroscopy (Kondo & Hirota 1970).In addition, divergences with previous results derived by microwave spectroscopy (Hirota 1979) are found for the trans-gauche energy difference.
In the present paper, highly correlated ab initio calculations are employed to explore the far-infrared region.A three-dimensional variational procedure depending on the three internal rotations, is employed.The model assumes the interconversion of the two gauche and the trans minima of the potential energy surface.It allows us to compute the 27 subcomponents of the torsional energies simultaneously and to obtain gauche-gauche and gauchetrans energy gaps.The model allows us to predict transitions between gauche and trans levels.This study maps the large amplitude region up to 500 cm −1 that contains the fundamentals and overtones.
To ensure that predictions are useful for further experimental studies, the level of theory has been tested using available experimental data.It provides very accurate geometries and rotational constants.Other infrared regions are also described using second-order perturbation theory to analyze the effects derived from other vibrational modes.The splitting subcomponents are classified using the irreducible representations of the molecular symmetry group (MSG).New predicted properties can help the spectroscopic analysis of the rotationalvibrational spectra in the gas phase and further detections of vibrationally excited isopropyl alcohol.

Quantum Chemical Approach
The geometry of the two conformers of isopropyl alcohol, as well as, the three-dimensional potential energy surface depending on the three torsional coordinates, were determined using explicitly correlated coupled-cluster theory with single and double substitutions augmented by a perturbative treatment of triple substitutions, CCSD(T)-F12 (Adler et al. 2007;Knizia et al. 2009) implemented in MOLPRO (2022) using the default options.A core-valence correlation consistent basis set, cc-pCVTZ-F12 (denoted by CVTZ-F12 in this paper), optimized for accurately describing core-core and core-valence correlation effects with the explicitly correlated F12 method, was employed (Hill et al. 2010).
Anharmonic properties for all the vibrational modes have been computed using vibrational second-order perturbation theory (VPT2; Barone 2004) as implemented in GAUSSIAN 16 revision C.01 (Frisch et al. 2016).The vibrational corrections of the rotational constants, as well as, those of the threedimensional potential energy surface were obtained using Möller-Plesset theory (MP2; Møller & Plesset 1934) and the aug-cc-pVTZ basis set (denoted by AVTZ; Kendall et al. 1992).
The rotational constants were computed using the CCSD(T)-F12 equilibrium parameters, A e , B e , and C e , and the MP2/ AVTZ vibrational contribution to the rotational constants, ΔB vib , computed from the VPT2 α ir vibration-rotation interaction parameters.The following equation, proposed and verified in previous studies (Toumi et al. 2022), summarizes the procedure: For the 3N a -6 vibrational modes (N a = number of atoms), J = 0 energy levels have been computed using the following equation for all the modes: where ω i represents the CCSD(T)-F12 harmonic fundamentals and x ij are the MP2/AVTZ anharmonic constants computed using VPT2.

Variational Procedure of Reduced Dimensionality
VPT2 theory has been developed for semirigid systems, which show a unique minimum in the potential energy surface.Then, it does not represent the proper theory for the three modes responsible for the nonrigidity.The low-lying torsional energies were computed variationally by solving a threedimensional Hamiltonian for J = 0 (Senent 1998a(Senent , 1998b)): 3 This rovibrational Hamiltonian defined for J = 0, can be derived by assuming the separability of the three large amplitude motions with respect to the other vibrational modes; B qiqj (θ 1 , θ 2 , α) are the kinetic energy parameters; the effective potential is the sum of three terms: , ,  , ,  , , , 4 q q a q q a q q a q q a = + ¢ + where V(θ 1 , θ 2 , α) represents the ab initio potential energy surface, V′(θ 1 , θ 2 , α) is the Podolsky pseudopotencial and V ZPVE (θ 1 , θ 2 , α), the zero-point vibrational energy correction.
The Hamiltonian of Equation (3) is solved variationally using symmetry-adapted Fourier as trial functions.This requires the analysis of the isopropyl alcohol symmetry.

The Symmetry of Isopropyl Alcohol
The two conformers of isopropyl alcohol gauche and trans can be classified in the C 1 and C s point groups, respectively.However, if the feasible large amplitude vibrations that interconvert the minima are considered, isopropyl alcohol can be classified in the G 18 MSG of methyl amine (Senent & Smeyers 1996) and N-methyl acetamide (Ohashi et al. 2004).This group contains six symmetry species.The nondegenerate A1 and A 2 are symmetric and antisymmetric with respect to the double-switch-exchange operation.The pseudo-degenerate representations E 1 and E 2 include a complex conjugate pair of one-degenerate representations E 1a and E 1b , E 2a and E 2b .E 3 is a two-fold degenerate representation and G contains a complex conjugate pair of two-fold degenerate representations G 1a , G 1b , G 2a , and G 2b .

Energetic and Structure of Isopropyl Alcohol
The energetics of the two conformers, gauche (G) and trans (T), of isopropyl alcohol are represented in Figure 1.The gauche structure corresponds to a double minimum.As the torsion of the two methyl groups interconvert nine equivalent minima, the ground electronic state potential energy surface presents a total of 3 × 9 = 27 quasi-identical minima.Table 1 summarizes the most characteristic parameters G (or G′) represents the most stable gauche geometry.Using CCSD(T)-F12 theory, the energy difference between both equilibrium structures has been computed to be E = 97.7 cm −1 .If the MP2/AVTZ anharmonic zero-point vibrational energy is considered, the energy difference decreases until E ZPVE = 80.2 cm −1 .To characterize the conformers, we use three coordinates θ 1 , θ 2 , and α, which are defined using linear combination of dihedral angles (Szalay et al. 2002): The atom labels are defined in Figure 2 devoted to the most symmetric conformer trans.To define α, an X phantom atom has been located in the plane defined by the three carbon atoms, in the C1C2C3 angle bisector pointing to the oxygen atom.In further sections of this paper, we describe the variational procedure in three dimensions for which θ 1 , θ 2 , and α represent the three independent coordinates.Given the G 18 symmetry of the molecule, the energy of isopropyl alcohol obeys the following equation: 1 2 2 1 q q a q q a = ---Figures 1 and 3 represent one-dimensional cuts of the ground electronic state potential energy surface, V(α) and V(θ), (θ = θ 1 or θ 2 ).As is observed for other properties (such as the rotational constant or the dipole moments), the methyl torsional barrier V 3 is very similar in both conformers.It has been   computed to be 1169 cm −1 (gauche) and to be 1278 cm −1 (trans).To obtain the curves of Figure 3, the α coordinate has been frozen at its value in the minimum energy geometries.The order of magnitude of V 3 is similar to other species showing two interacting methyl groups, such as dimethyl-ether 1996), or dimethyl sulfoxide (V 3 = 965 cm −1 ; Senent et al. 2015).Given the values of the barrier, very small gaps between methyl torsional splittings can be expected (∼0.0001 cm −1 ).However, the barriers of Figure 1, V OH (g → g) = 476 cm −1 and V OH (g → t) = 425 cm −1 restricting the interconversion of the conformers gauche and trans, allow for anticipating nonnegligible energy differences between subcomponents of the low-lying vibrational energies.The computed barriers are coherent with the measurements of Kondo & Hirota (1970), who, assuming the threefold periodicity for the OH torsion, determined the barrier to be V t = 587.6 cm −1 .

Rovibrational Parameters
Table 2 collects the ground vibrational state rotational parameters computed using a multipart approach summarized in Equation (1).This equation has been proposed and verified in previous studies (Toumi et al. 2022).The basis is that the most contributed term is determined using a very expensive computational tool.
In Table 2, calculated parameters are compared with those derived by Maeda et al. (2006) from spectrum measurements in the millimeter-and submillimeter-wave region.In this experimental work, the fitting and predictions for isopropanol were accomplished using the programs SPFIT and SPCAT (Pickett 1991).Differences between the present computed and experimental data of Maeda et al. (2006; ΔB 0 = B calc -B exp ) are very small in the gauche form (ΔA 0 = 3.7 MHz; ΔB 0 = 1.8 MHz; ΔC 0 = 1.75 MHz) and are smaller in the trans form (ΔA 0 = 1.3 MHz; ΔB 0 = 0.9 MHz; ΔC 0 = 1.0 MHz).The centrifugal distortion constants of Table 2 are parameters of the asymmetrically reduced Watson Hamiltonian in the Ir representation (Watson 1968).
In Table 3, all the fundamental transitions computed using the multipart approach summarized in Equation (2), are ordered according to the symmetry criteria of the C 1 gauche structure.Emphasized in bold are the transitions for which displacements to higher frequencies (↑) and to lower frequencies (↓) by Fermi resonances are relevant.The test of Fermi resonances was performed by diagonalizing a matrix built with the energy levels corresponding of excitations of one or two vibrational modes (diagonal terms), and the cubic force field (out-diagonal terms).Displacements have important effects on the stretching modes involving hydrogen atoms.
Assignments and observations of 19 infrared spectral bands measured in the gas phase by Green (1963) are also summarized in Table 3.In these assignments, gauche and trans transitions are not discriminated because the coexistence of different quasi-energetic conformers was not observed.Two torsional fundamentals were estimated to lye at 230 cm −1 (hydroxyl torsion) and 260 cm −1 (methyl torsion) from the values of the torsional barriers.The assignments of two prominent Q branches at 209 and 234 cm −1 to the OH torsional fundamentals of the trans and gauche conformers of Inagaki et al. (1973) are also shown in Table 3.
For a set of 17 transitions, which includes torsions, there is reasonable agreement between our calculations and the assignments of Green (1963).However, there are two exceptions that concern the two bands observed at 1072 and 955 cm −1 , assigned by Green (1963) to the C = O stretching and to the CH 3 rock, respectively.In these two cases, the ab initio results refute the observed band interpretation.According to our computations, the higher frequency band at 1078 cm −1 (gauche) and at 1074 cm −1 (trans) corresponds to the HOC bending, whereas the band at 959 cm −1 (gauche) and 962 cm −1 (trans) corresponds to the C = O stretching.On the basis of the present calculations, we propose a new assignment for these two bands, as indicated in Table 3.
For the gauche conformer, the three low-lying anharmonic fundamentals have been computed to be ν 30 = 216 cm −1 (CH 3 torsion), ν 29 = 256 cm −1 (CH 3 torsion), and ν 28 = 265 cm −1 (OH torsion).The order is different for the trans conformer, where the torsional fundamentals lie at ν 30 = 217 cm −1 (a″ CH 3 torsion), and ν 28 = 238 cm −1 (a″ OH torsion), and ν 29 = 256 cm −1 (a′ CH 3 torsion).For some fundamentals, i.e., the two antisymmetric torsions ν 30 and ν 28 , the normal modes cannot be interpreted as OH or CH 3 torsional local modes due to the string coupling.The normal modes cannot be fairly correlated to local modes.It has to be considered that the VPT2 theory has been developed for semirigid species and the resulting anharmonic torsional fundamentals are not fully reliable.This is the reason why, in the next section, we describe a variational analysis of the far-infrared region.

The Far-infrared Region: Low-lying Vibrational Energy Levels
The ab initio potential energy surface V(θ 1 , θ 2 , α) (see Equations (3) and 4) was computed from the CCSD(T)-F12/ CVTZ-F12 energies of 100 geometries defined for a grid of values of three dihedral angles, taking the symmetry into consideration: H9C3C1C2 0 , 90 , 180 , 90 H12O4C1X 0 , 45 , 90 , 135 , 180, 45 , 90 , 135 .7 In all the 100 structures selected for the three internal coordinates, the 3N a -3-6 coordinates were optimized at the CCSD(T)-F12/CVTZ-F12 level of theory.The surface was fitted to a triple Fourier series transforming as the totally symmetric representation of the isopropyl alcohol MSG: å q q a q q q q a q q q q a q q q q a q q q q a q q q q a q q q q a q q q q a = ´+ + + The linear fit reached R 2 = 0.9999 and σ = 2.73 cm −1 .Formally identical equations were employed for the pseudopotential V′(θ 1 , θ 2 , α) and the vibrational correction V ZPVE (θ 1 , θ 2 , α).Although the pseudopotential represents a negligible contribution, the zero vibrational energy correction can displace the final energy levels and the amount that can reach 10 cm −1 .V ZPVE was computed at the MP2/AVTZ level of theory and within the harmonic approximation using all the 100 geometries (Császár et al. 2004).
The expansion coefficients of V eff (θ 1 , θ 2 , α) are provided in Table A1.The coefficients of the coupling terms cos3θ 1 cos3θ 2 (+72.526cm −1 ) and sin3θ 1 sin3θ 2 (−63.161cm −1 ) describe the interactions between the two methyl groups.The last one represents the main contribution to the gap between the two methyl torsional fundamentals.These coefficients are 10 times lower than the one of the uncoupled term cos3θ 1 +cos3θ 2 (711.1 cm −1 ), which represents the main contribution to the barrier.
The term coslα describes the OH torsion.The most contributed one is cos3α (230.714cm −1 ) due to the shape of the OH torsional potential approaching C 3v tops.This was predicted by Kondo & Hirota (1970) who assumed a threefold symmetry for the internal rotation of the OH group.
Formally identical equations to Equation (6) were employed for the kinetic parameters ( ) B , , q q 1 2 i j q q , which were computed using the 100 selected geometries (Senent 1998a(Senent , 1998b)).The expansion coefficients (most of them are very small) are provided in Table A2 to help the reproducibility of the present study.The most contributed terms are the A ccc 000 coefficients: A 000 (B aa ) = A 000 (B bb ) = 5.6683 cm −1 , A 000 (B ab ) = −0.1395cm −1 , and A 000 (B ac ) = A 000 (B bc ) = −0.1055cm −1 (a, b = CH 3 torsion; c = OH torsion).Table 4 shows low-lying energy levels.To compute variationally the torsional energy levels, symmetry-adapted Fourier series were employed as trial functions.The convergence requires at least a basis set containing 20 cosines and 19 sines to describe each methyl torsion, and 26 cosines and 25 sines to describe the OH torsion.This leads to a Hamiltonian matrix with 77571 × 77571 elements.As symmetry-adapted trial functions are employed, the matrix factorizes in eight submatrices whose dimensions are 4631 (A 1 ), 4303 (A 2 ) in the case of the two nondegenerate representations, 8632 (E 1 ), 8606 (E 2 ) for the pseudo-degenerate species, 8619 (E 3a ) and 8619 (E 3b ) for the pseudo-degenerate representations, and 17238 (G a ) and 17238 (G b ) for the double-pseudo-degenerate species.For these last representations, contracted basis sets were used to reduce dimensionality (Boussessi & Senent 2020).
The energies in Table 4 are referred to as the threedimensional zero vibrational energy (385.722cm −1 ).The VPT2 vibrational energies corresponding to neglected modes g Derived from the torsional barriers (Green 1963).
As the potential energy surface shows 27 quasi-equivalent minima, the low vibrational energy levels split into 27 subcomponents.The levels are classified using symmetry and three quanta, ν 30 , ν 29 and ν OH .
The OH torsion splits the levels into three components (ν 30 , ν 29 , ν OH ), (ν 30 , ν 29 , ν OH + ) and (ν 30 , ν 29 , OH n -).Due to the methyl torsion, each component splits into nine nondegenerate and degenerate subcomponents whose symmetries are: A i (i = 1 or 2), G, E i (i = 1 or 2), and E 3 .The scheme of Figure 4 represents the nondegenerate subcomponents of the ground vibrational state, the fundamental and overtones.The figure helps to understand the relative positions of the levels with respect to the OH barriers.
Table 4 shows energy levels up to 500 cm −1 .A total of 207 energies (fundamentals, overtones, and combination bands), whose classification is complex, lie in this region.Below 300 cm −1 , ∼90 states involved in the fundamental transitions have been found.Overtones and combination bands are included in Table 4 because they allow for obtaining anharmonic constants if they are needed.To interpret the results, several procedures based on the properties of the threedimensional wave functions have been employed (probability integrals, expectation values of one-dimensional Hamiltonians, coefficients of the contracted basis sets, etc.; Boussessi & Senent 2020).The energies have been assigned to the minima, by computing probability integrals using the three-dimensional wave functions.To assign the quanta ν 30 , ν 29 , and ν OH , the expectation values of a one-dimensional Hamiltonian Ĥ(α) derived from Equation (5) allow us to distinguish the OH torsional excitations.To ensure a good assignment, the computation of the nondegenerate energies has been achieved using a contracted basis set for the OH coordinate (Boussessi & Senent 2020).
Finally, the torsional fundamental transitions are shown in Table 5.The 27 components of the OH fundamental have been found to lie between 210.348 and 216.799 cm −1 .The subcomponents of the two methyl fundamentals are found to lie between 237.525 and 243.551 cm −1 and between 260.148 and 262.156 cm −1 .
In the work of Maeda et al. (2006), where different treatments are applied to the gauche and trans conformers, the possibility of unassigned gauche-trans transitions is suggested.As we consider both conformers together, in Table 5, the corresponding band centers are computed, although it can be expected that IR intensities will be less strong for gauche-trans transitions than for the gauche-gauche and trans-trans transitions, due to the overlap of the wave functions of the connected levels.If these transitions are considered, weak bands can be observed between 134.702 and 344.253 cm −1 .
The vibrational energies were employed to estimate the vibrational partition function Q v : The E vib energies corresponding to excitations of the medium-and the low-amplitude modes were obtained using Equation (2).At 100 K, Q vib augments 4 times if the nondegenerate levels of Table 4 are employed in Equation (9), instead of the VPT2 excited torsional energies.

Astrophysical Implications
Aliphatic alcohols can be precursors of lipids in the primitive Earth.The search for biological molecules containing a crescent number of carbon atoms, which is the aim of many astrophysical observations, reached the detection of n-and i-propanol in Sgr B2(N) (Belloche et al. 2022) and n-propanol in G+0.693-0.027(Jiménez-Serra et al. 2022).The relative abundances of the two isomers in different sources may be related to the lack of physical and chemical data.
A large amount of species detected in the ISM are nonrigid biological molecules, which, as propanol, show isomers and conformers.In general, excluding diatomic species, 30% of all interstellar molecules have observed isomeric counterparts (Lovas et al. 2019).The presence of conformers separated by low-energy barriers can have implications in spectroscopy and reactivity.The assignments of their rotational spectra require models contemplating the nonrigidity.The application of radiative transfer models for the interpretation of astrophysical observations entails the computation of partition functions.Widicus Weaver et al. (2005) showed that excited vibrational state contributions to the partition functions are an important consideration when determining the column density of a molecule with low-lying torsional states.In isopropanol, the consideration of the complete set of low-lying states multiplies by 4 the vibrational partition function at 100 K.
Highly correlated theory, accurate enough to distinguish between isopropanol conformer rotational constants, allows for mapping low-lying torsional states.The employed variational procedure is useful for mapping the very dense far-infrared region, which cannot be done experimentally or using a low level of theory.Below 300 cm −1 , we found ∼90 torsional energy levels.Excitations of the OH torsion, which is responsible for the g → t process, show the lowest energies.Figure 4 evidences that the g → t and t → g interconversion processes are restricted by very low barriers of 325 and 250 cm −1 computed from the ground vibrational energies (0 0 0).Given the feasibility of the processes, both conformers gand -t must be treated together.Over 325 cm −1 , it is not possible to discriminate conformers.

Conclusions
In this paper, spectroscopic parameters of isopropyl alcohol, a volatile organic compound present in the Earth and in extraterrestrial atmospheres, are derived from highly correlated ab initio calculations (CCSD(T)-F12).The ground vibrational state rotational constants have been employed to evaluate the quality of the level of theory.The differences between computed and experimental data (ΔB 0 = B calc -B exp ) are small for the gauche form (ΔA 0 = 3.7 MHz; ΔB 0 = 1.8 MHz; ΔC 0 = 1.75 MHz) and smaller for the trans form (ΔA 0 = 1.3 MHz; ΔB 0 = 0.9 MHz; ΔC 0 = 1.0 MHz).

Appendix
The Appendix (Tables A1 and A2) contains expansion coefficients for the parameters of the 3D-Hamiltonian.

Figure 1 .
Figure 1.Energy profile of the interconversion process of the isopropyl alcohol conformers.

Figure 4 .
Figure 4. Nondegenerate subcomponents of the low-lying energy levels.In red are emphasized the OH torsional excitations.

Table 2
Vibrational Ground State Rotational Constants, and Quartic and Sextic Centrifugal Distortion Constants (in MHz a ) a Parameters of the asymmetrically reduced Watson Hamiltonian in the Ir representation.

Table 5
A i Components of Transitions Connecting the Low-energy Levels (in cm −1 )