A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution

Article abstract of DOI:10.1002/chem.202003270

Phenylmethanimine is an aromatic imine with a twofold relevance in chemistry: organic synthesis and astrochemistry. To tackle both aspects, a multidisciplinary strategy has been exploited and a new, easily accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. The combination of this formation pathway, based on the thermal decomposition of hydrobenzamide, with a state-of-the-art computational characterization of phenylmethanimine laid the foundation for its first laboratory observation by means of rotational electric resonance spectroscopy. Both E and Z isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. A further characterization has been carried out by nuclear magnetic resonance spectroscopy, showing the feasibility of this synthetic approach in solution. The temperature dependence as well as possible mechanisms of the thermolysis process have been examined.

Full text of DOI:10.1002/chem.202003270

Accepted Article
Accepted Article
Title: A journey from thermally-tunable synthesis to spectroscopy of
phenylmethanimine in gas-phase and solution
Authors: Alessio Melli, Simone Potenti, Mattia Melosso, Sven Herbers,
Lorenzo Spada, Andrea Gualandi, Kevin G. Lengsfeld, Luca
Dore, Philipp Buschmann, Pier Giorgio Cozzi, Jens-Uwe
Grabow, Vincenzo Barone, and Cristina Puzzarini
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202003270
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01/2020

10.1002/chem.202003270
Chemistry - A European Journal
A Journey from Thermally-Tunable Synthesis to
Spectroscopy of Phenylmethanimine in Gas
Phase and Solution
Alessio Melli1,2, Simone Potenti1,2, Mattia Melosso2, Sven
Herbers3, Lorenzo Spada*1,2, Andrea Gualandi2, Kevin G.
Lengsfeld3, Luca Dore2, Philipp Buschmann3, Pier Giorgio Cozzi*2,
Jens-Uwe Grabow*3, Vincenzo Barone*1, and Cristina Puzzarini*2
1Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
2Dipartimento di Chimica “Giacomo Ciamician”, Universita` di Bologna, Via Selmi
2, 40126 Bologna, Italy
3Institut fu¨r Physikalische Chemie und Elektrochemie, Gottfried Wilhelm Leibniz
Universita¨t Hannover, Callinstraße 3A, 30167 Hannover, Germany
E-mail: lorenzo.spada@sns.it; piergiorgio.cozzi@unibo.it;
jens-uwe.grabow@pci.uni-hannover.de; vincenzo.barone@sns.it;
cristina.puzzarini@unibo.it
1
Abstract
2
3
Phenylmethanimine is an aromatic imine with a twofold relevance in chem-
istry: organic synthesis and astrochemistry. To tackle both aspects, a
multidisciplinary approach has been exploited and a new, easily accessible
synthetic approach to generate stable imine-intermediates in the gas phase
and in solution has been introduced. The combination of this formation
pathway – based on the thermal decomposition of hydrobenzamide – with
a state-of-the-art computational characterization of phenylmethanimine
laid the foundation for its first laboratory observation by means of rota-
tional electric resonance spectroscopy. Both E and Z isomers have been
accurately characterized, thus providing a reliable basis to guide future as-
tronomical observations. A further characterization has been carried out
by nuclear magnetic resonance spectroscopy, showing the feasibility of this
synthetic approach in solution. The temperature dependence as well as
possible mechanisms of the thermolysis process have been examined.
4
5
6
7
8
9
10
11
12
13
14
15
16 1 Introduction
17 In the last few decades, astrochemistry –the research field focused on the chem-
18 ical composition and evolution of ordinary matter in space– has flourished dy-
19 namically: after being born as a niche sector it grew up into a field of broad
1
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20 interest. Its birth is relatively recent because, for many years, the interstellar
21 medium (ISM) –the space between star systems in a galaxy– has been con-
22 sidered too hostile to bear any chemical complexity. However, with the rise of
23 radioastronomy (in the 1960s), it became evident that the ISM harbors a diverse
24 collection of interesting polyatomic (both organic and inorganic) molecules.[1,2]
25 Since then, a large number of molecular species has been detected.[3] Among
26 them, those having a prebiotic character are of particular interest because of
27 their key role in several mechanisms leading to the emergency of life. Imines
28 belong to this category.
29
While their astrobiological relevance has not yet unambiguously proven,
30 imines are known to play major roles in many chemical processes. They have
31 been recognized as crucial intermediates in organic synthesis, due to their exten-
32 sive use in the preparation of N-containing compounds,[4] and as naturally or
33 biologically active molecules.[5] Because of peculiar and extreme conditions, the
34 chemical reactivity in space is very different from the terrestrial counterpart,
35 and unstable species (even ions and radicals) can survive long. As a conse-
36 quence, reactive species such as some N-H imines, whose isolation is difficult on
37 Earth, can be observed in the ISM,[6–10] with the prerequisite of a successful
38 laboratory spectroscopy characterization. With the chemistry in the ISM being
39 new and intriguing, severe difficulties arise when interstellar molecules are to be
40 produced by “terrestrial” organic procedures for their characterization, e.g. by
41 molecular spectroscopy. Indeed, the instability of these species often requires
42 that they are directly generated inside the spectrometer using harsh conditions
43 and rather unpredictable techniques such as pyrolysis or electric discharge. We
44 have tried to reconcile these difficulties by developing a different and easier syn-
45 thetic route, the thermally tunable formation of imines, which has been tested
46 both in the gas phase and in solution.
47
The subject of this work is the case study of phenylmethanimine (PMI),
48 which is a non-standard reactive molecule of potential astrochemical relevance.
+
49 Due to the large amount of hydrogen (in the form of H, H2, and also H+/H3
)
50 available in the ISM, hydrogenation is an efficient process[11] and leads to the
51 formation of saturated or partially saturated molecules.[12] On this ground, hy-
52 drogenation of benzonitrile (BN) in the ISM can yield PMI, an interesting imine
53 in terms of chemical complexity and molecular evolution. This gained attention
54 because of the recent detection of BN in the cold-core Taurus Molecular Cloud
55 1 (TMC-1), which provided the unequivocal proof of the presence of benzene in
56 that cold environment, and –more generally– in the ISM.[13] In turn, benzene
57 is the building block of polycyclic aromatic hydrocarbons (PAHs), which are
58 recognized (but spectroscopically not proven) to be important constituents of
59 small dust grains in interstellar clouds and are believed to play a key role in
60 the chemical evolution in space from both a catalytic and protective point of
61 view.[14]
62
Despite its appeal as a good approach for imine synthesis in ISM, on Earth,
63 nitrile reduction using molecular hydrogen usually requires catalytic conditions
64 which are not compatible with a single hydrogenation process. Catalytic pro-
65 cesses directly lead to the saturated derivative, i.e. the corresponding amine,
2
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10.1002/chem.202003270
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66 or to a mixture of primary-, secondary- and even tertiary-amines via reaction
67 of the N-H imine intermediates.[15,16] This is due to the high reactivity of the
68 imine intermediate.[17–21] The N-H imine derivatives of aliphatic or aromatic
69 aldehydes are known to be much less stable, and only in a few cases they have
70 been successfully isolated or characterized.[22,23] Indeed, the literature is rich of
71 examples supporting the instability and reactivity of these imines. The chem-
72 ical labile nature of N-H imines, and the difficulties related to their isolation,
73 resulted in the employment of imines possessing various substituents on the
74 nitrogen atom (N-substituted imines). Therefore, the activating or protecting
75 groups introduced on the nitrogen atom need to be removed after the synthetic
76 endeavor. Based on the concept of atom economy, N-unsubstituted imines would
77 be ideal, since deprotection steps, to obtain the final products, are not required.
78 Unfortunately, unlike N-substituted imines, the approaches to N-unsubstituted
79 imines are limited to (and, in many cases, were proposed as) unstable, not char-
80 acterized intermediates.[24] The direct reaction with ammonia and aldehydes is
81 not a suitable synthetic procedure for such imines. N-H imines can be accessed
82 by the controlled hydrolysis of N-metalloimines or silylimines.[25] Another sim-
83 ple access to N-H imines involves available azides precursors with the removal of
84 N2 and subsequent migration of hydrogen, under catalytic conditions.[26] Nev-
85 ertheless, all these approaches are impracticable in order to generate N-H imines
86 for rotational spectroscopy investigations in gas phase. Indeed, such context in-
87 creased the relevance of a thermally tunable approach from a simple precursor
88 for the generation of N-H imines and their spectroscopic investigation. Such
89 approaches avoid critical issues such as: (i) demanding experimental setups, i.e.
90 subtle control of reaction conditions; (ii) trapping procedures leading to imine
91 complexes; (iii) use of transition metal catalysts; (iv) unfeasible interface with
92 spectroscopic techniques.
93
The latter aspect is of particular interest to our investigation since, in order
94 to set up and validate our synthetic route, high-resolution spectroscopic tech-
95 niques offer the highest specificity: rotational electric resonance (RER) spec-
96 troscopy in the gas phase and nuclear magnetic resonance (NMR) spectroscopy
97 in solution. Since our strategy is based on two key points, namely (i) the use of
98 stable and affordable organic precursors and (ii) the generation of the unstable
99 and reactive imines through simple conditions, it provides a suitable way to
100 produce imines directly inside the spectrometer.
101
In this manuscript, we report a full account of our successful endeavor, which
102 has led –for the first time– to a complete and accurate spectroscopic characteri-
103 zation of PMI using RER, which is prerequisite for its identification in the ISM
104 by means of radioastronomy. It required a multidisciplinary effort combining
105 organic synthesis and molecular spectroscopy, the latter supported and guided
106 by computational chemistry.
3
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107 2 Results and discussion
108 Spectroscopic characterization of PMI. To guide RER experiments, a
109 state-of-the-art quantum-chemical (QC) characterization has been performed.
110 Using the density functional theory (DFT, see supporting information, SI, for
111 details), a preliminary scan of the potential energy surface has been carried
112 out, which located two isomers (E and Z, referring to the relative position of
113 the phenyl group with respect to the imine hydrogen, see Figure 1). Subse-
114 quently, an accurate structural determination has been obtained by resorting to
115 the so-called “cheap” composite scheme[27] (hereafter denoted as ChS), whose
116 denomination refers to the limited computational cost in spite of its accuracy.
117 The ChS approach is expected to◦provide results with an accuracy of about
[27]
˚
118 0.001-0.002 A for bonds and 0.1-0.2 for angles.
This approach has also been
119 used to derive accurate electronic energies, which are given in Figure 1. While
120 structural parameters and a detailed description of the ChS model are pro-
121 vided in the SI, this figure points out that the E isomer is about 6 kJ mol−1
122 more stable than the Z species. TSE-Z has a quasi-linear C–N–H geome-
123 try, the isomerization process occurring in the molecular plane, similarly to
124 what already observed for other imines such as ethanimine (ETI)[28] and C-
125 cyanomethanimine (CCMI).[29] Although different computational schemes have
126 been applied, a qualitative comparison between the relative energies is possi-
127 ble. A smaller energy difference between the two isomers was obtained for ETI
128 and CCMI (2.8 kJ mol−1 and 2.0 kJ mol−1, respectively[28,29]), while a similar
129 isomerization barrier was found (about 116 kJ mol−1 for ETI,[28] ∼111 kJ mol−1
119.9
(112.2)
TS
100.0
50.0
6.5
(6.3)
0.0
(0.0)
Z
0.0
E
Isomerization coordinate
Figure 1: Stationary points of PMI, relative electronic energies at the ChS
level. Values within parentheses are augmented by harmonic B2PLYP zero-
point energy corrections.
4
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Chemistry - A European Journal
130 for CCMI,[29] and about 112 kJ mol−1 for PMI).
Equilibrium rotational constants have been straightforwardly derived from
131
132 the ChS equilibrium structure, while the required vibrational corrections have
133 been evaluated from DFT anharmonic force-field calculations (thereby using
134 B3LYP,[30,31] see SI for details). First-order properties, such as dipole moment
135 components and nuclear quadrupole coupling constants, have been computed
136 using, in the framework of DFT, the double-hybrid B2PLYP functional[32] (see
137 SI for details). This level of theory has also been employed for deriving cen-
138 trifugal distortion constants.
139
The gas-phase characterization of PMI has been performed in the 3-26 GHz
140 range, using the COBRA-type (Coaxially Aligned Beam Resonator Arrange-
141 ment) Fourier Transform Microwave Spectrometer (FTMW), described in de-
142 tails elsewhere.[33] A nozzle valve, filled with hydrobenzamide (HBA) and heated
143 to 85 ◦C, was used to vaporize it and, at the same time, to thermally-decompose
144 it, thus producing PMI in the gas phase. HBA was synthesized, following the
145 procedure described in literature,[17,34–36] by condensation of benzaldehyde and
146 ammonium hydroxide solution and obtained as white solid, with its identity and
147 purity being confirmed by NMR analysis (see the SI for details).
148
Given the high accuracy of the QC calculations performed (the relevant spec-
149 troscopic parameters are collected in Table 1), the computational prediction of
150 the rotational spectrum was expected to match the accuracy[37] required for
151 an unequivocal identification of the E and Z isomers of phenylmethanimine in
152 the gas phase. Indeed, it was straightforward to successfully identify and as-
153 sign more than one hundred transition frequencies for both PMI isomers. Their
154 unique pattern (hyperfine structure), due to quadrupole coupling splitting orig-
155 inated by the presence of 14N nucleus (I = 1), is shown in Figure 2 for both
4 ← 3
0.12
0.30
5 ← 4
0.10
5 ← 4
3 ← 2
0.08
4 ← 3
0.20
0.06
3 ← 2
0.04
0.10
0.02
0.00
10309.8
0.00
10310.0
10310.2
10310.4
10736.4
10736.6
10736.8
10737.0
Frequency/MHz
Frequency/MHz
Figure 2: The 41,4 ← 31,3 transition of E-phenylmethanimine (top panel) and
the 40,4 ← 30,3 transition of the Z isomer (bottom panel), denoting rotational
transitions JK
← JK0 0 0 . In both spectra, the reported quantum numbers
a,Kc
a,Kc
refer to the nuclear quadrupole coupling scheme F ← F0, F = J + I and the
line pairs becoming split by the Doppler effect are made evident.
5
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Table 1: Comparison between experimental (exp.)a and calculated (best theo.)
spectroscopic parameters (expressed in MHz, dipole moment components are
given in debye).
E-phenylmethanimine
exp.
Z-phenylmethanimine
exp.
best theo.
best theo.
A
B
C
5217.29202(11)
1565.283633(28)
1204.540307(14)
5.775(11)
5216.95
1564.68
1204.05
5.57
5200.81278(16)
1548.969349(92)
1194.842313(78)
5.643(35)
5200.42
1548.76
1194.30
5.28
DJ × 105
DK × 104
DJK × 104
d1 × 105
d2 × 106
1.5χaa
7.65(11)
1.678(37)
−1.6941(55)
−3.162(23)
1.5271(14)
7.07
1.81
−1.63
−3.00
1.61
7.06b
7.06
1.57
1.445(47)
−1.542(44)
−1.51
−2.68
−6.05
−0.55
3.09
−2.68b
−5.8871(23)
−0.4954(15)
0.25(χbb − χcc
)
−1.70630(38)
−1.83
|µa|
0.77
1.66
|µb|
0.15
0
|µc|
# lines
0
c
180
4.56
0.65
118
1.38
0.69
rms × 103
d
σ
a S reduction, Ir representation. Standard errors are reported within parentheses.
b Fixed at the corresponding computed value.
c Distinct frequency lines.
d Dimensionless quantities.
156 PMI isomers. Furthermore, we note that each rotational transition appears
157 as a doublet due to the Doppler effect arising from a supersonic jet expan-
158 sion. For the most stable E-PMI, the spectral analysis has been extended in
159 the 83-100 GHz, thus allowing the improvement of the spectroscopic parame-
160 ters, using a frequency-modulation millimeter-wave (FM-mmW) spectrometer
161 (see ref. 38 for a detailed description). In this experiment, a different pro-
162 duction method of PMI has been employed, as reasoned below. Indeed, PMI
163 was produced by means of flash vacuum pyrolysis (FVP)[39,40] of a sample of
164 α-methylbenzylamine (890 ◦C).
165
The spectroscopic parameters derived from a weighted non-linear fit of the
166 observed transitions, performed using the Pickett’s CALPGM suite of programs,[41]
167 are reported in Table 1. An excellent agreement between the computed and ex-
168 perimental values can be noted, with an average error of 0.03% on the effective
169 rotational constants and a maximum discrepancy below 0.05%. Only small de-
170 viations have also been found for the quartic centrifugal distortion and nuclear
171 quadrupole coupling constants. Since rotational and quadrupole coupling con-
172 stants are strongly tied to the molecular structure, the very good agreement
6
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173 gives further confidence in the reliability and accuracy of the computed geome-
174 tries.
175
While the full list of the rotational transitions included in the fit is given in
176 the SI, we state here that the root mean square error (rms) is smaller than 5 kHz,
177 with a standard deviation (σ) slightly below unity, i.e. achieving experimental
178 accuracy.
179 Thermal decomposition of HBA. The successful generation and charac-
180 terization of PMI in the gas phase raised questions about the mechanism of
181 its formation and the possible extension of the methodology to the condensed
182 phase (i.e. solution) and, more generally, to other N-unsubstituted imines (thus
183 avoiding more expensive and/or more chemically demanding procedures). For
184 this reason, the thermal behavior of HBA has been explored from room tem-
185 perature to 100/110 ◦C, both in the gas phase by means of RER and in solution
1
186 by H-NMR.
187
In particular, the JK
= 50,5 ← 40,4 rotational transition of four molec-
a,Kb
188 ular species, namely the BA, BN and the E and Z isomers of PMI, has been
189 recorded with the COBRA-FTMW spectrometer. To provide a reliable com-
190 parison between the intensity of the transitions, the power of the microwave
Figure 3: Spectra of the JK
= 50,5 ← 40,4 transition for the four molecular
a,Kb
species investigated (PMI: phenylmethanimine; BA: benzaldehyde, BN: ben-
zonitrile). The huge abundance of BA in the gas phase leads to out-of-chart
transitions at any temperature.
7
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191 excitation pulse has been scaled according to the value of dipole moment com-
192 ponent along the a-axis and a 2000 averaging signal acquisition has been chosen.
193 The recording has been started ten minutes after the temperature set up. The
194 recorded spectra (see Figure 3) reveal the presence of BA at all temperatures,
195 while clearly distinguishable signals of PMI isomers are observed only above
196 80 ◦C. BN lines appear at the same temperature. For both PMI isomers and
197 BN, it is noted that rising the temperature increases the intensity of the tran-
198 sitions, thus improving the signal-to-noise ratio (S/N). Although not shown in
199 Figure 3, the 12321.0 MHz transition of the water dimer[42] was observed for
200 each temperature increment. This confirms the steady presence of water in the
201 experimental apparatus, which very likely plays a key role in the investigated
202 hydrolytic process.
203
An important note on the relative populations of the two PMI isomers is
204 deserved. In principle, this information should be derivable from the spectra of
205 Figure 3; however, the E-PMI transition there reported lies very close to the
206 strong transition of BA, this clearly modifying the baseline of the E-PMI tran-
207 sition. As a consequence, the signal-to-noise ratios of Z-PMI and E-PMI lines
208 cannot be compared, thus preventing the derivation of the relative populations.
209 We can only comment that, during the recording of the spectra, we noticed
210 that the intensities of both isomers were comparable. This is probably due to
211 a co-operation of population distribution and differences in the electric dipole
212 moment components.
213
As to the NMR characterization, according to the available literature data,
214 PMI shows two distinct sets (each with two doublets) of signals for the imino hy-
215 drogen, with different coupling constants (J = 16 Hz for the E isomer, J = 25
216 Hz for the Z one).[26] In our analysis, the sample was heated to the desired
217 temperature and the 1H-NMR spectrum was acquired after 10 minutes to avoid
218 thermalization processes during the acquisition (a complete account of the ex-
219 periment is provided in the SI). The formation of both BA and E-PMI was ob-
220 served as a consequence of the step-by-step temperature increasing from 25 ◦C
221 to 110 ◦C (see Figure 4). However, no spectroscopic clues denoting the possi-
222 ble formation of BN or Z-PMI were observed, even when the experiments were
223 conducted at higher temperature. As far as Z-PMI is concerned, however, our
224 experimental resolution did not allow for unequivocally excluding its presence.
225 This suggests that the two experiments (RER and NMR) are characterized by
226 either different hydrolytic mechanisms or the availability of additional pathways
227 in the case of gas-phase measurements. To confirm the absence of BN in solution
228 after thermal treatment of HBA, a trace amount of the former was added to the
229 NMR samples previously heated at 110 ◦C and the spectrum was again recorded.
230 The analysis revealed the presence of new signals (doublet at 7.68 ppm, over-
231 lapping with a BA triplet; doublet at 7.63 ppm; triplet at 7.50 ppm, overlapping
232 with a HBA multiplet), due to the presence of BN and previously absent. Hence,
233 no BN is formed by hydrolysis of HBA in solution (see SI). This might suggest
234 that its generation occurs through pathways that rely on metallic catalytic sur-
235 faces, such as fast dehydrogenation of PMI.[43,44] Indeed, it has been proved that
236 the metal catalysis can take place in the nozzle head in the RER experiment,[45]
8
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Figure 4: Top panel: NMR signals at different temperatures for HBA and its
thermolysis products. Bottom panel: Temperature-dependence of the normal-
ized integral values (residual solvent peak was fixed to 10) of the NMR signals. In
both panels, red triangles refer to HBA CHN (8.63-8.60 ppm), cyan hexagons to
aminal HBA (6.06-5.99 ppm), blue upside down triangles to PMI ortho-protons
(7.77-7.76 ppm), green diamonds to BA para-proton (7.68 ppm), black squares
to BA CHO (10.08-10.02 ppm) and grey circles to PMI CHN (8.70 ppm). The
aminal HBA signal is not shown in the top panel for the sake of clarity.
9
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Ph
BA
O
PMI
NH
H2O
H2
Ph
NH
CN
N
N
N
NH2
COBRA-FTMW
nozzle
conditions
BN
PMI
I
HBA
Scheme 1: Proposed mechanism for HBA hydrolysis.
237 while this is not the case within the NMR tube.
Figure 4 shows the changes –upon heating– of the diagnostic NMR signals
238
239 (top) and the temperature-dependence of their normalized integral values with
240 the respective linear regressions (bottom). Such analysis was performed after
241 an arbitrary assignment to the integral value of the residual peak of 1,1,2,2-
242 tetrachloroethane-D2 (fixed to 10). The negative-slope traces correspond to the
243 starting material, i.e. HBA, while the positive-slope traces are attributable to
244 forming species. As expected, the signal relative to the two CHN protons of
245 HBA (red trace) decreases twice as fast as the single-proton aminal signal (cyan
246 trace).
247
Two signals (i.e. green and black traces) have almost superimposed trends
248 in terms of both slope and normalized integral values, thus suggesting that the
249 respective protons belong to the same species. Indeed, they are compatible
250 with the CHO and para-proton (triplet) of BA, respectively, thus featuring a
251 1:1 integration relationship. The other two signals (i.e. blue and gray traces)
252 feature a 2:1 integration relationship. In particular, the singlet at 8.70 ppm (gray
253 trace) is compatible with an iminic CHN proton, while the doublet at 7.77 ppm
254 (blue trace) is compatible with the iminic ortho-protons. Furthermore, the grey
255 trace has a slope which is similar to those observed for the aldehydic signals,
256 thus suggesting that the generation processes of the two species are somehow
257 interconnected. The fact that BA appears to be produced before PMI suggests
258 that the involved hydrolytic mechanism proceeds with the initial formation of
259 BA, followed by the formation of PMI starting from other transient species.
260
Here, we propose a putative hydrolytic pathway (Scheme 1) which is compat-
261 ible with the aforementioned observations and with previously proposed mecha-
262 nisms.[46–48] As mentioned above, the additional pathway leading to the forma-
263 tion of BN starting from PMI during the measurements in the microwave range
264 is supposed to be a dehydrogenation.
265
In our opinion, the following key points need to be analysed to explain the
266 proposed mechanism: (i) the role of water; (ii) the role of the metallic surface in
267 the COBRA-FTMW spectrometer with respect to the glass surfaces acting in
268 the FM-mmW experiment. Within the former context, water is expected to play
269 a crucial role: the hydrolytic pathway is supposed to proceed as long as water is
270 available. In fact, the amount of water detected by means of low-frequency RER
10
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271 experiment as well as the residual water of the deuterated solvent is believed
272 to be sufficient to observe the formation of the main species arising from HBA
273 hydrolysis, i.e. BA at room temperature and PMI at higher temperatures.
274
On the other hand, the FM-mmW experiment can provide further clues
275 for a deeper understanding of the hydrolytic mechanism. At first, a tentative
276 generation of PMI in the gas phase has been carried out by thermolysis of HBA.
277 The solid was placed in a glass tube and heated up to 100 ◦C, also ensuring
278 a uniform heating along the path to the absorption cell. While heating the
279 sample up, a portion of the spectrum around 85.5 GHz was scanned in the
280 attempt of detecting two strong transitions of E-PMI, as predicted by our low-
281 frequency measurements. No signal attributable to PMI (nor BN) was found.
282 Typically, the whole glass apparatus of the FM-mmW spectrometer is pumped
283 continuously, thereby removing water, although its residual presence cannot be
284 rule out. However, no metallic surfaces are available in the instrument, thus
285 leading to the formulation of two hypotheses for the lack of formation of PMI in
286 the FM-mmW experiment: (i) water is not available in a sufficient amount to
287 hydrolyze HBA; (ii) the metal catalysis is mandatory to obtain PMI from HBA
288 in gas phase.
289
To verify the reliability of spectral predictions and therefore rule out the pos-
290 sibility of false negatives, we adopted a different production method, i.e. FVP.
291 By pyrolysis of two possible precursors of PMIi, i.e. benzylamine and α-methyl
292 benzylamine, through dehydrogenation or elimination of CH4, respectively, a
293 small set of 27 transitions (belonging only to E-PMI) could be measured, with
294 the latter precursor leading to the highest S/N of the spectra. Conversely, the
295 use of N-methyl benzylamine gave no signals ascribable to the presence of PMI.
296 BN was found as a pyrolysis co-product, as proven by recording its rotational
297 transitions. While the PMI signal reached its maximum intensity by setting the
298 furnace temperature to 890 ◦C, the intensity of BN transitions kept increasing
299 up to 1200 ◦C. This confirms the prevalence of BN at higher temperatures, in
300 agreement with the low-frequency RER experiment.
301
Although a thorough analysis of the mechanisms taking place in the FVP
302 process is beyond the scope of this work, the FM-mmW experiment proved the
303 reliability of the centimeter-wave RER measurements and their extrapolation at
304 higher frequencies, but it left some unexplored areas concerning PMI formation
305 from HBA in the gas phase if no metallic surfaces are available. As mentioned
306 above, such possibilities could be further explored going beyond simple thermal
307 conditions. Several attempts to identify the Z isomer have been carried out, but
308 no signal ascribable to it was found. This might suggest that only the E isomer
309 is generated by flash vacuum pyrolysis, but we did not investigate further this
310 aspect in the present study.
iFor a vacuum dynamic preparation of PMI, see ref. 49.
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311 3 Conclusions
312 An easy and affordable approach based on hydrobenzamide thermolysis is pre-
313 sented, which ensures to obtain phenylmethanimine both in gas-phase and in
314 solution as confirmed by rotational electric resonance (RER) and 1H-nuclear
315 magnetic resonance (NMR) spectroscopy experiments, respectively. A detailed
316 structural and energetic description of phenylmethanimine has been carried out
317 by resorting to composite schemes for accurate results. This paved the way
318 for the registration and analysis of the microwave spectrum of both E- and Z-
319 phenylmethanimine, leading to their first laboratory identification. This work
320 is a prerequisite for the possible radioastronomical detection of these species in
321 the interstellar medium, relying on accurate rotational rest frequencies. First,
322 in view of the strong chemical connection between benzonitrile and phenyl-
323 methanimine, an astronomical search in the region Taurus Molecular Cloud
324 (TMC-1) is suggested. Finally, owing to the thorough analysis of RER and NMR
325 spectra at different temperatures, a possible mechanism of phenylmethanimine
326 formation by thermal tuning of hydrobenzamide, in which water is thought to
327 play a crucial role, is also proposed.
328 Associated Content
329 Supporting Information. Details on computational methodologies and spec-
330 troscopic RER and NMR techniques. Experimental details for the synthesis of
331 HBA. Characterization of HBA. Copies of NMR spectra of HBA at different
332 temperature. NMR check for BN absence in solution. List of all assigned RER
333 transitions. Optimized structural parameters for the stationary points.
334 Acknowledgements
335 This work has been supported by MIUR ‘PRIN 2015’ (Grant Number 2015F59J3R),
336 ’PRIN 2017’ (Grant Number 2017A4XRCA) and by the University of Bologna
337 (RFO funds). Partial support by the Italian Space Agency (ASI; ‘Life in Space’
338 project, N. 2019-3-U.0) is also acknowledged. Further support was obtained by
339 the Deutsche Forschungsgemeinschaft (DFG, GR-1344) and the Land Nieder-
340 sachsen. KGL acknowledges a Fonds der Chemie (FCI) scholarship. The authors
341 gratefully thank Dr. A. Petroli and Prof. A. Mazzanti for their assistance in
342 the variable temperature NMR experiment and the SMART@SNS Laboratory
343 (http://smart.sns.it) for providing high-performance computing facility.
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427 Table of Contents Graphic
Figure 5: Table of Contents Graphic
A multidisciplinary approach involving organic synthesis and high-resolution
428
429 molecular spectroscopy has led to the definition of a thermally-tunable forma-
430 tion route for imines. Its application to phenylmethanimine combined with an
431 integrated rotational spectroscopy - quantum chemistry protocol has enabled
432 the full characterization of the latter, thus providing the means for future as-
433 tronomical observations.
16
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