Oxazine Ring-Related Vibrational Modes of Benzoxazine Monomers Using Fully Aromatically Substituted, Deuterated, 15N Isotope Exchanged, and Oxazine-Ring-Substituted Compounds and Theoretical Calculations

Article abstract of DOI:10.1021/acs.jpca.7b05249

Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm^-1 band in infrared spectroscopy (IR). Historically, this band was assigned to the C-H out-of-plane bending of the benzene to which the oxazine ring is attach

Full text of DOI:10.1021/acs.jpca.7b05249

Subscriber access provided by BOSTON UNIV
Article
Oxazine Ring-Related Vibrational Modes of Benzoxazine Monomers
15
Using Fully Aromatically Substituted, Deuterated, N Isotope Exchanged
and Oxazine-Ring-Substituted Compounds and Theoretical Calculation
Lu Han, Daniela Iguchi, Phwey S. Gil, Tyler Heyl, Victoria M Sedwick,
Carlos R. Arza, Seishi Ohashi, Daniel J. Lacks, and Hatsuo Ishida
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05249 • Publication Date (Web): 27 Jul 2017
Downloaded from http://pubs.acs.org on July 27, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry A is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Oxazine Ring-related Vibrational Modes of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Benzoxazine Monomers Using Fully Aromatically
1
5
Substituted, Deuterated, N Isotope Exchanged and
Oxazine-ring-substituted Compounds and
Theoretical Calculation
1
1
2
1
1
1
Lu Han , Daniela Iguchi , Phwey Gil , Tyler Heyl , Victoria M. Sedwick , Carlos R. Arza , Seishi
1
Ohashi , Daniel J. Lacks , Hatsuo Ishida*
2
1
1
Department of Macromolecular Science and Engineering, Case Western Reserve University,
Department of Chemical and Biomolecular Engineering, Case Western Reserve University,
2
Cleveland, OH, 44106, United States.
*
Corresponding Author: E-mail: hxi3@cwru.edu
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 2 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT
-1
Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm band
in infrared spectroscopy (IR). Historically, this band was assigned to the C-H out-of-plane
bending of the benzene to which the oxazine ring is attached. This study shows that this band is a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
mixture of the O-C stretching of the oxazine ring and the phenolic ring vibrational modes.
2
Vibrational frequencies of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-a) and 3-(tert-
butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-t) are compared with isotope-exchanged and
15
all-substituted compounds. Deuterated benzoxazine monomers,
N-isotope exchanged
benzoxazine monomers, and all-substituted benzoxazine monomers without aromatic C-H
groups are synthesized and studied meticulously. The various isotopic-exchanges involved
deuteration around the benzene ring of phenol, selective deuteration of each CH in the O-CH -N
2
2
(2) and N-CH -Ar (4) positions on the oxazine ring, or simultaneous deuteration of both
2
1
positions. The chemical structures were confirmed by H nuclear magnetic resonance
1
spectroscopy ( H NMR). The IR and Raman spectra of each compound are compared. Further
1
5
analysis of N isotope-exchanged PH-a indicates the influence of the nitrogen isotope on the
band position, both experimentally and theoretically. This finding is important for
polymerization studies of benzoxazines that utilize vibrational spectroscopy.
2
ACS Paragon Plus Environment

Page 3 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
. INTRODUCTION
Polybenzoxazine is a relatively new and commercialized thermoset resin obtained by
polymerizing 1,3-benzoxazine. Recently, polybenzoxazine received much attention because of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1,2
their many unique and desirable properties, such as near-zero shrinkage upon polymerization,
3
low flammability, lower surface free energy than polytetrafluoroethylene without having
4
fluorine atoms, extremely flexible molecular design capability,
,5
6-11
which even allows the use
1
of phenols and amines from natural renewable resources, and other excellent mechanical
2
13-15
1
6-17
and thermal properties.
-1
Benzoxazines are characterized by a broad and intense band at 960-900 cm in the FT-IR
spectrum. This band gradually disappears when benzoxazine is heated to the polymerization
7
,18
temperature. For fully polymerized samples, this band completely disappears.
It has been
reported by Dunkers and Ishida that the disappearance of this band does not exactly coincide, but
-1
that there is a time lag between the intensity change of the 1498 cm band (which is directly
-1 19,20
associated with the polymerization) and the band of current interest near 960-900 cm .
-1
While the circumstantial evidence clearly points to the strong association of the 960-900 cm
band with benzoxazine polymerization, the nature of this band has not been well understood.
Historically, this band of 1,3-benzoxazine was assigned to the C-H out-of-plane bending
vibrations of benzene to which the oxazine is attached. Since the band assignment of this mode
in early days of benzoxazine research many years ago, no rigorous effort was made to understand
this mode in detail despite its very important role in polymerization studies of benzoxazine resin.
Therefore, the goal of this study is to understand the nature of this band by systematically
studying the FT-IR and Raman spectra of the compounds specifically designed to answer
questions related to the group that is involved and nature of the vibration.
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 4 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Benzoxazines are usually synthesized by one-pot Mannich condensation of phenol, amine and
2
1
formaldehyde. Various benzoxazine monomers can be synthesized by using different phenolic
derivatives and primary amines, which are commercially available. To selectively substitute the
oxazine ring, however, there is a limitation for one-pot synthesis. In order to selectively
substitute or deuterate the 2-position of the oxazine ring, 2-step synthesis should be conducted by
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2-23
starting with ortho-hydroxyl substituted benzaldehyde.
By using different benzaldehyde or
formaldehyde, different oxazine-substituted benzoxazine monomers can be synthesized. On the
other hand, 4 position deuterated benzoxazine can be obtained by hydrolyzing the all deuterated
24
benzoxazine, followed by closing the ring using normal paraformaldehyde.
The approaches taken in the current paper can be largely classified into two steps. First, in order
to test if the band is directly from the aromatic C-H groups, a series of fully aromatically
substituted benzoxazines were designed and synthesized. Two benzoxazine monomers, bis- and
tris-oxazine as shown below in Figure 1, are designed to evaluate the role of the aromatic C-H on
the benzoxazine modes. If the band of interest is due to the aromatic C-H, these monomers
should not show this characteristic band. Although PG-a and PG-anis are the same with respect
to the substitution structure, it was nonetheless synthesized to perturb the overlapped vibrational
bands.
4
ACS Paragon Plus Environment

Page 5 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Figure 1. Chemical structure of all-substituted benzoxazines.
Isotopic substitution is a useful technique to assign FT-IR or Raman bands because isotopically
substituted molecules are different from the unsubstituted molecule only by weight but not the
electronic structure, leading to different vibrational frequencies from the same vibrational
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
modes. PH-a-phenol-d , shown in Figure 2, was designed to further evaluate if the
5
4
characteristic mode is aromatic related.
The second goal of the current paper is to examine, if the band of current interest is not aromatic
C-H related, then which group of the oxazine it is most strongly related to. Compounds PH-a-
2oxazine-d , PH-a-4oxazine-d and PH-a-oxazine-d , are designed to evaluate if the band is
2 2 4
related to oxazine 2, 4 position CH . By using symmetric and asymmetric deuteration, the
2
specific position of the CH that is related to the band can be examined. Furthermore, if the band
2
is related to the skeletal oxazine ring mode rather than oxazine CH modes, the position of the
2
vibration, either O-C-N or N-C-Ar can be evaluated.
PH-d-a was designed to study whether the modes are associated with aniline part.
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 6 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Chemical structure of PH-a series.
The following four compounds (shown in Figure 3) are the corresponding tert-amine versions of
the monomers shown above. By using these compounds, the overlapped aromatic amine proton
information can be further eliminated. Although some information obtained from the aniline-
series and tert-amine series are redundant, it is nonetheless useful to make sure that the
information obtained is reproducible.
6
ACS Paragon Plus Environment

Page 7 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Figure 3. Chemical structure of PH-t series.
1
5
15
N-exchanged PH-a monomer, PH-a- N, will further allow if the characteristic mode is related
to the N-C group or Ar-O/Ar-C groups.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Finally, by changing the substitution position of methyl group on the oxazine ring (shown in
Figure 4.), further strong evidence can be obtained if the band belongs either to the N-CH -Ar or
2
1
O-CH -N group and the conclusion derived from the N-isotope compound can be verified.
5
2
Figure 4. Chemical structure of methyl group substituted benzoxazine series.
2. EXPERIMENTAL
2.1. Materials
2,5-dimethylresorcinol (95%), hexylamine (99%), phloroglucinol ( ≥ 99%), formaldehyde
solution (37 wt. % in H O, contains 10-15% ethanol as stabilizer), aniline (99%), glacial acetic
2
acid, p-anisidine (≥99%), salicylaldehyde (98%), paraformaldehyde-d (98 atom % D), phenol (
2
≥99%), phenol-d (99 atom % D), hydrochloric acid (37.5%), distilled water, ammonium
6
1
hydroxide (25% solution), tert-butyl amine (98%), aniline- N (98 atom % N), 2-acetylphenol (
5
15
≥
98%), acetaldehyde(≥99%), and aniline-2,3,4,5,6-d (98 atom % D) were purchased from
5
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 8 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Sigma-Aldrich. Chloroform-d was obtained from Cambridge Isotope Laboratories, Inc.
Paraformaldehyde (96%) was obtained from Acros Organics. Magnesium sulfate anhydrous,
sodium hydroxide, toluene (99.9%), ethanol (200 proof), hexane (4.2% various methylpentanes),
ethyl acetate, acetone, formic acid (88%), chloroform (99.9%), dichoromethane (99.9%), and
diethyl ether (99.9%) were purchased from Fisher Scientific.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2.2. Characterization
1
H nuclear magnetic resonance (NMR) spectra were acquired on a Varian Oxford AS600 at a
1
proton frequency of 600 MHz. The average number of transients for H measurement was 64. A
1
relaxation time of 10 s was used for the integrated intensity determination of H NMR spectra.
Fourier transform infrared (FT-IR) spectra were obtained using a Bomem Michelson MB100
FTIR spectrometer, which was equipped with a deuterated triglycine sulfate (DTGS) detector
-1
and a dry air purge unit. Coaddition of 64 scans was recorded at a resolution of 4 cm . The
intensity of the strongest absorption band was controlled to 0.7-1 for accuracy, by changing the
thickness of the film casted on the KBr plate.
2.3. Synthesis
2.3.1. Synthesis of 3,7-dihexyl-5,10-dimethyl-3,4,7,8-tetrahydro-2H,6H-benzo[1,2-e:5,4-
e']bis([1,3]oxazine) (abbreviated as 25DMR-h)
Toluene (5 mL) was added to a mixture of 2, 5-dimethylresorcinol (0.100 g, 0.72 mmol),
paraformaldeyde (0.109 g, 3.62 mmol) and hexylamine (0.147 g, 1.45 mmol) in a 15 mL round
bottom flask. The mixture was magnetically stirred for 6 h under reflux. The completed reaction
product was washed with 1N NaOH three times and water three times. The product was dried
8
ACS Paragon Plus Environment

Page 9 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
over magnesium sulfate anhydrous overnight. The solution was filtered to remove the salt. After
drying the filtrate, the product was dried in a vacuum oven to obtain a light brown powder.
1
Yield: 73%. H-NMR (600 MHz, CDCl , ppm): δ 0.81-0.87 (m, 6H, -CH ), 1.23-1.32 (m, 16H,
3
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CH -(CH ) - CH ), 1.89 (s, 3H, -CH ), 1.98 (s, 3H, -CH ), 2.69 (t, 4H, N-CH -(CH2) ), 3.83 (s,
4
2
2 4
3
3
3
2
4H, Ar-CH -N), 4.76 (s, 4H, N-CH -O).
2
2
2.3.2. Synthesis of 3,7,11-triphenyl-3,4,7,8,11,12-hexahydro-2H,6H,10H-benzo[1,2-e:3,4-
e':5,6-e'']tris([1,3]oxazine) (abbreviated as PG-a)
Ethanol (5 mL) was added to a mixture of phloroglucinol (0.100 g, 0.79 mmol), formaldehyde
(0.483 g, 5.95 mmol), aniline (0.222 g, 2.38 mmol) and a catalytic amount of glacial acetic acid
in a 15 mL round bottom flask. The mixture was magnetically stirred for 10 h at room
temperature. The product precipitated from the reaction solvent and was collected by filtration.
1
The product was dried in a vacuum oven to obtain a white powder. Yield: 74%. H-NMR (600
MHz, CDCl , ppm): δ 4.43 (s, 6H, Ar-CH -N), 5.27 (s, 6H, N-CH -O), 6.91 (d, 3H, ArH), 7.08
3
2
2
(d, 6H, ArH), 7.24 (t, 6H, ArH).
2.3.3.
Synthesis of 3,7,11-tris(4-methoxyphenyl)-3,4,7,8,11,12-hexahydro-2H,6H,10H-
benzo[1,2-e:3,4-e':5,6-e'']tris([1,3]oxazine) (abbreviated as PG-anis)
Ethanol (5 mL) was added to a mixture of phloroglucinol (0.100 g, 0.79 mmol), formaldehyde
(0.483 g, 5.95 mmol), p-anisidine (0.293 g, 2.38 mmol) and a catalytic amount of glacial acetic
acid in a 15 mL round bottom flask. The mixture was magnetically stirred for 8 h at room
temperature. The product precipitated from the reaction solvent and collected by filtration. The
1
product was dried in a vacuum oven to obtain white powder. Yield: 92%. HNMR (600 MHz,
9
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 10 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CDCl , ppm): δ 3.73 (s, 9H, CH ), 4.34 (s, 6H, Ar-CH -N), 5.18 (s, 6H, N-CH -O), 6.78 (d, 6H,
3
3
2
2
ArH), 7.02 (d, 6H, ArH).
2.3.4. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as PH-a)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Toluene (3 mL), phenol (0.100 g, 1.06 mmol), aniline (0.094 g, 1.01 mmol) and
paraformaldehyde (0.076 g, 2.53 mmol) were added to a 10 mL round bottom flask. The mixture
was magnetically stirred and heated under reflux for 6 h. The completed reaction was washed
with 1N NaOH three times and water three times. The product was dried over magnesium sulfate
anhydrous. The material was purified in an alumina neutral column with hexane and ethyl
acetate (9:1) as the eluent. The material was recrystallized from hexane. The product was a white
1
crystal. Yield: 85%. H-NMR (600MHz, CDCl , ppm): δ 4.64 (s, 2H, Ar-CH -N), 5.38 (s, 2H, N-
3
2
CH -O), 6.84-6.85 (d,1H, ArH), 6.90-6.92 (t, 1H, ArH), 6.94-6.97 (t, 1H, ArH), 7.02-7.04 (d,
2
1H, ArH) 7.13-7.14 (m, 3H, ArH), 7.27-7.30 (m, 2H, ArH).
2.3.5. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-5,6,7,8-d (abbreviated as
4
PH-a-phenol-d4)
Toluene (3 mL), phenol-d (0.100 g, 1.01 mmol), paraformaldehyde (0.072 g, 2.40 mmol), and
6
aniline (0.089 g, 0.96 mmol) were added to a 10 mL round bottom flask. The mixture was
magnetically stirred and heated under reflux for 6 h. The completed reaction was washed with
1N NaOH three times and water three times. The product was dried over magnesium sulfate
anhydrous. The material was purified in an alumina neutral column with hexane and ethyl
acetate (9:1) as the eluent. The material was recrystallized from hexane. The product was a white
1
crystal. Yield: 88%. H-NMR (600 MHz, CDCl , ppm): 4.62 (s, 2H, Ar-CH -N), 5.34 (s, 2H, N-
3
2
CH -O), 6.90-6.92 (t, 1H, ArH) 7.08-7.10 (d, 2H, ArH), 7.23-7.26 (t, 2H, ArH).
2
10
ACS Paragon Plus Environment

Page 11 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
2
.3.6. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2-d (abbreviated as
2
PH-a-2oxazine-d2)
Ethanol (40 mL) was added to a mixture of salicylaldehyde (1.000 g, 8.18 mmol) and aniline
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
0.762 g, 8.18 mmol) in a 100 mL round bottom flask. The mixture was magnetically stirred for
2 h under reflux. After cooling to room temperature, NaBH (1.549 g, 40.8 mmol) was added
4
once an hour, three times. The solution was precipitated in water and dried to yield a white
powder 2-((phenylamino)methyl)phenol (2-pamp). The powder was dried in vacuum oven to
eliminate the remaining water. Yield: 94%.
Toluene (3 mL), 2-pamp (0.467 g, 2.34 mmol), and paraformaldehyde-d (0.093 g, 2.93 mmol)
2
were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated
under reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water
three times. The product was dried over magnesium sulfate anhydrous. The material was purified
in an alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The material was
1
recrystallized from hexane. The product produced was a white crystal. Yield: 83%. H-NMR
(600 MHz, CDCl , ppm): δ 4.62 (s, 2H, Ar-CH -N), 6.79-6.80 (d, 1H, ArH), 6.86-6.88 (t, 1H,
3
2
ArH), 6.91-6.93 (t, 1H, ArH), 6.99-7.01 (d, 1H, ArH), 7.09-7.11 (t, 3H, ArH), 7.23-7.26 (t, 2H,
ArH).
2
.3.7. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2,4,4-d (abbreviated as
4
PH-a-oxazine-d4)
Toluene (15 mL) was added to a mixture of phenol (0.500 g, 5.32 mmol), paraformaldehyde-d2
(0.405 g, 5.00 mmol), and aniline (0.472 g, 5.10 mmol) in a 50 mL round bottom flask. The
mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction was
11
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 12 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
washed with 1N NaOH three times and water three times. The product was dried over
magnesium sulfate anhydrous. The material was purified in an alumina neutral column with
hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from hexane. The
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
product produced was a white crystal. Yield: 85%. H-NMR (600 MHz, CDCl , ppm): δ 6.73-
3
6.750 (d, 1H, ArH), 6.81-6.83 (t, 1H, ArH), 6.94-6.96 (d, 1H, ArH), 7.03-7.06 (t, 3H, ArH),
7.18-7.21 (t, 2H, ArH).
2.3.8. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-4,4-d (abbreviated as
2
PH-a-4oxazine-d2)
PH-a-oxazine-d (0.500 g, 2.32 mmol) was dissolved in 25 mL of n-propanol, hydrochloric acid
4
(37.5% w/w, 7.14 mL) and distilled water (3.57 mL) was added to the solution. The mixture was
o
magnetically stirred and heated under reflux for 2 h. After cooling to 0 C, 25% ammonium
hydroxide (17.87 mL) was added to the solution. The mixture was magnetically stirred for 1 hour
at room temperature. After extracting with ethyl acetate 3 times, a white powder 2-
((phenylamino)methyl-d )phenol (2-pamd p) was obtained after drying the solvent in the
2 2
vacuum oven. Yield: 95%.
Toluene (3 mL), 2-pamd p (0.259 g, 1.29 mmol), and paraformaldehyde-d (0.048 g, 1.61
2
2
mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred and
heated under reflux for 6 h. The completed reaction product was washed with 1N NaOH three
times and water three times. The product was dried over magnesium sulfate anhydrous. The
material was purified in an alumina neutral column with hexane and ethyl acetate (9:1) as the
eluent. The material was recrystallized from hexane. The product produced was a white crystal.
1
Yield: 82%. H-NMR (600 MHz, CDCl , ppm): δ 5.34 (s, 2H, N-CH -O), 6.78-6.80 (d, 1H,
3
2
12
ACS Paragon Plus Environment

Page 13 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
ArH), 6.86-6.88 (t, 1H, ArH), 6.91-6.93 (t, 1H, ArH), 6.99-7.00 (d, 1H, ArH), 7.08-7.11 (t, 3H,
ArH), 7.23-7.26 (t, 2H, ArH).
2.3.9. Synthesis of 3-(phenyl-d )-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as PH-
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
d-a)
Toluene (3 mL), phenol (0.100 g, 1.06 mmol), paraformaldehyde (0.076 g, 2.53 mmol) and
aniline-2,3,4,5,6-d (0.099 g, 1.01 mmol) were added to a 10 mL round bottom flask. The
5
mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction was
washed with 1N NaOH three times and water three times. The product was dried over
magnesium sulfate anhydrous. The material was purified in an alumina neutral column with
hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from hexane. The
1
product produced was a white crystal. Yield: 90%. H-NMR (600 MHz, CDCl , ppm): 4.66 (s,
3
2H, Ar-CH -N), 5.39 (s, 2H, N-CH -O), 6.83-6.85 (d, 1H, ArH) 6.90-6.93 (t, 1H, ArH), 7.04-
2 2
7.05 (d, 1H, ArH), 7.13-7.16 (t, 1H, ArH).
2.3.10. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as Ph-
t)
Toluene (3 mL) was added to a mixture of phenol (0.100g, 1.06 mmol), tert-butyl amine (0.074
g, 1.01 mmol) and paraformaldehyde (0.076 g, 2.40 mmol) in a 10 mL round bottom flask. The
mixture was magnetically stirred for 2 h under reflux. The yellowish solution was washed with
1N NaOH three times and water three times. The solution was dried over magnesium sulfate
anhydrous overnight. The material was purified in an alumina neutral column with hexane as the
1
eluent. The purified compound was a transparent liquid. Yield: 77%. H-NMR (600 MHz,
13
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 14 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CDCl , ppm): δ 1.22 (s, 9H, CH ), 4.130 (s, 2H, Ar-CH -N), 5.00 (s, 2H, N-CH -O), 6.76-6.77
3
3
2
2
(d, 1H, ArH), 6.85-6.87 (t, 1H, ArH), 6.99-7.00 (d, 1H, ArH), 7.08-7.10 (t, 1H, ArH).
2.3.11.
Synthesis
of
3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-5,6,7,8-d4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(abbreviated as PH-t-phenol-d4)
Toluene (3 mL) was added to a mixture of phenol-d (0.100 g, 1.00 mmol), tert-butyl amine
6
(0.070 g, 0.95 mmol) and paraformaldehyde (0.071 g, 2.40 mmol) in a 10 mL round bottom
flask. The mixture was magnetically stirred for 2 h under reflux. The yellowish solution was
washed with 1N NaOH three times and water three times. The solution was dried over
magnesium sulfate anhydrous overnight. The material was purified in an alumina neutral column
1
with hexane as the eluent. The purified compound was a transparent liquid. Yield: 78%. H-
NMR (600 MHz, CDCl , ppm): δ 1.184 (s, 9H, CH ), 4.097 (s, 2H, Ar-CH -N), 4.962 (s, 2H, N-
3
3
2
CH -O).
2
2.3.12. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2-d (abbreviated
2
as PH-t-2oxazine-d2)
Ethanol (20 mL) was added to a mixture of salicylaldehyde (1.000 g, 8.19 mmol) and aniline
(0.599 g, 8.19 mmol) in a 50 mL round bottom flask. The mixture was magnetically stirred for 2
h under reflux. After cooling to room temperature, NaBD (1.549 g, 40.95 mmol) was added
4
once an hour for three times. The solution was precipitated in 100 mL water by adjusting the pH
to 8. Dichloromethane was used to extract the intermediate 2-((tert-butylamino)methyl)phenol
(2-tbamp) from the aqueous layer three times. The solution was dried over magnesium sulfate
anhydrous overnight. After filtering the salt, the filtrate was dried in a vacuum oven. A white
crystal was obtained. Yield: 62%.
14
ACS Paragon Plus Environment

Page 15 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Toluene (3 mL), 2-tbamp (0.200 g, 1.12 mmol), and paraformaldehyde-d (0.045 g, 1.40 mmol)
2
were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated
under reflux for 2 h. The yellowish solution was washed with 1N NaOH three times and water
three times. The solution was dried over magnesium sulfate anhydrous overnight. The material
was purified in an alumina neutral column with hexane as the eluent. The purified compound
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
was a transparent liquid. Yield: 77%. H-NMR (600 MHz, CDCl , ppm): δ 1.20 (s, 9H, CH ),
3
3
4.11 (s, 2H, Ar-CH -N), 6.75 (d, 1H, ArH), 6.83-6.85 (t, 1H, ArH), 6.95-6.97 (d, 1H, ArH), 7.05-
2
7.08 (t, 1H, ArH).
2.3.13.
Synthesis
of
3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2,4,4-d4
(
abbreviated as PH-t-oxazine-d4)
Toluene (3 mL) was added to a mixture of phenol (0.100g, 1.06 mmol), tert-butyl amine (0.074
g, 1.01 mmol) and paraformaldehyde-d (0.081 g, 2.53 mmol) in a 10 mL round bottom flask.
2
The mixture was magnetically stirred for 2 h under reflux. The yellowish solution was washed
with 1N NaOH three times and water three times. The solution was dried over magnesium
sulfate anhydrous overnight. The material was purified in an alumina neutral column with
1
hexane as the eluent. The purified compound was a transparent liquid. Yield: 76%. H-NMR
(
600 MHz, CDCl , ppm): δ 1.174 (s, 9H, CH ), 6.72-6.73 (d, 1H, ArH), 6.81-6.83 (t, 1H, ArH),
3
3
6.95-6.96 (d, 1H, ArH), 7.04-7.07 (t, 1H, ArH).
1
5
.3.14. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-3- N (abbreviated as
2
15
N-PH-a)
Toluene (3 mL), phenol (0.110g, 1.12 mmol), paraformaldehyde (0.080 g, 2.66 mmol) and
1
aniline- N (0.100 g, 1.06 mmol) were added to a 10 mL round bottom flask. The mixture was
5
15
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 16 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
magnetically stirred and heated under reflux for 6 h. The completed reaction product was washed
with 1N NaOH three times and water three times. The product was dried over magnesium sulfate
anhydrous. The material was purified in an alumina neutral column with hexane and ethyl
acetate (9:1) as the eluent. The material was recrystallized from hexane. The product produced
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
was a white crystal. Yield: 90%. H-NMR (600 MHz, CDCl , ppm): 4.67 (s, 2H, Ar-CH -N),
3
2
5.40 (s, 2H, N-CH -O), 6.84-6.86 (d, 1H, ArH) 6.91-6.94 (t, 1H, ArH), 6.95-6.98 (t, 1H, ArH),
2
7.05-7.06(d, 1H, ArH), 7.14-7.16 (d, 3H, ArH), 7.29-7.32 (t, 2H, ArH).
2.3.15. Synthesis of 4-methyl-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as
Ph-a-4oxazine)
Ethanol (20 mL) was added to a mixture of 2-acetylphenol (0.500 g, 3.67 mmol) and aniline
(0.342 g, 3.67 mmol) in a 50 mL round bottom flask. The mixture was magnetically stirred for 2
h under reflux. After cooling to room temperature, NaBH (0.695 g, 18.36 mmol) was added
4
once an hour for three times. The solution was precipitated in water and dried to yield a white
powder (2-(1-(phenylamino)ethyl)phenol (abbreviated as 2-1-pamep). The powder was dried in
a vacuum oven to get rid of the remaining water. Yield: 89%.
Toluene (3 mL), 2-1-pamep (0.400g, 1.88 mmol), and paraformaldehyde (0.070 g, 2.34 mmol)
were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated
under reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water
three times. The product was dried over magnesium sulfate anhydrous. The material was purified
in an alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The material was
1
recrystallized from hexane. The product produced was a white crystal. Yield: 83%. H-NMR
(
600 MHz, CDCl , ppm): δ 1.73-1.74 (d, 3H, CH-CH ), 4.68-4.71 (m, 1H, CH-CH ), 5.24-5.26
3
3
3
16
ACS Paragon Plus Environment

Page 17 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
(
d, 1H, N-CH -O), 5.43-5.45 (d, 1H, N-CH -O), 6.88-6.97 (m, 3H, ArH), 7.05-7.06 (d, 1H,
2
2
ArH), 7.13-7.15 (m, 3H, ArH), 7.27-7.30 (m, 2H, ArH).
2
.3.16. Synthesis of 2-methyl-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ph-a-2oxazine)
Toluene (3 mL), 2-pamp (0.400 g, 2.08 mmol), and acetaldehyde (0.093 g, 2.11 mmol) were
added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated under
reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water three
times. The product was dried over magnesium sulfate anhydrous. The material was purified in an
alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The product produced
1
was a viscous liquid. Yield: 83%. H-NMR (600 MHz, CDCl , ppm): δ 1.54-1.55 (d, 3H, CH-
3
CH ), 4.49-4.55 (q, 2H, Ar-CH -N), 5.62-5.65 (q, 1H, N-CH-Ar), 6.80-6.82 (d, 1H, ArH), 6.86-
2
3
6.88 (t, 1H, ArH), 6.96-7.00 (m, 2H, ArH), 7.08-7.10 (d, 2H, ArH), 7.12-7.14 (t, 1H, ArH), 7.22-
7.25 (m, 2H, ArH).
2.4 Computational Calculation
2
6,27
The Kohn-Sham formulation of density functional theory (DFT)
with the B3LYP
2
8-31
32-33
functional
and the 6-31G** basis set
is used. The 6-31G** basis set is used because the
inclusion of polarization functions on hydrogen atom orbitals significantly improved the
agreement between computed and experimental FT-IR spectra.
To carry out the vibrational analysis, the geometry of the molecule must first be optimized with
the same level of theory and basis set. The crystal structure of 3-phenyl-6,8-dichloro-3,4-
dihydrogen-1,3-benzoxazine (dichlorinated PH-a) has been reported by Liu and Gu using x-ray
17
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 18 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
34
diffraction (XRD) experiments. This molecule was reported to take a semi-chair conformation,
where the benzene-oxazine rings and the aniline rings have an interplanar angle of nearly 90°.
This structure was used as the starting structure of PH-a, the monomer of current interest, in the
geometry optimization calculations. The same initial structure was used for simulations of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
molecular isotopes, since molecular isotopes have nearly identical geometries. Avogadro, a
5
3
6
molecule editing software, was used to draw the initial structure of all simulated molecules.
3
7,38
Geometry optimization and vibrational analysis were performed with Gaussian 09 software.
3
9
GaussSum3 was used to extract the IR and Raman spectra from the Gaussian output. Computed
vibrational frequencies are often multiplied by a constant factor. The scaling partially
compensates for the errors associated with the assumption of harmonic vibrational modes
(whereas real vibrational modes are anharmonic) and with the electron correlation
approximations of the theory. The scaling factors are empirical; they are determined by
comparing experimental and calculated frequencies. To achieve this 15 strong bands from each
PH-a and 6 PH-a isotopes experimental FT-IR spectra were correlated to the theoretical results
obtaining the average scaling factor of 0.970 (See Table S1 in SI). This methodology is similar to
4
0
how Irikura et al. determine scaling factors.
41
Vibrational Energy Distribution Analysis 4 software (VEDA4) was utilized to perform the
potential energy distribution (PED) calculations. VEDA4 optimization procedure is designed so
that combinations of linearly independent internal coordinates are as close as possible to the
normal coordinates. VEDA4 was used to process the Gaussian output, yielding the full PED
matrix for analysis.
2.5 Deconvolution of Heavily Overlapped Bands
18
ACS Paragon Plus Environment

Page 19 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Peakfit V4.12 (Systat Software Inc.) was used for deconvolution of heavily overlapped IR bands,
-1
in particular the band in question around 960-900 cm . The range for curve resolving is set
based on the two minimum data points close to characteristic band in FT-IR spectrum. AutoFit
Peaks II Second Derivative method (Baseline: Linear, D2, total%=3.0, Smoothing%=1.00, Peak
type: Spectroscopy, Gaussian * Lorentz, AutoScan: Amp%=1.50, Vary Widths, Vary shape)
gave the initial peak positions. Repeated the Numerical Fitting routine until iteration is equal to
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
2,43
7.
3. Results and Discussion
In this paper, we have taken two different approaches to understand the nature of the prominent
-1
IR band around 960-900 cm which was previously assigned as the C-H out-of-plane mode of
benzene ring to which oxazine is attached.
3.1 Results
3.1.1. All-substituted benzoxazine monomers
Three all-substituted benzoxazine monomers, have been synthesized according to Scheme 1.
The first monomer is a bisoxazine which is abbreviated as 25DMR-h and is derived from 2, 5-
dimethylresorcinol and hexylamine. Others are trisoxazines which are abbreviated as PG-a or
PG-anis. These are derived from phloroglucinol with aniline.
19
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 20 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1. Synthesis scheme of all substituted benzoxazine.
1
In order to verify if the synthesis was successful, H NMR spectra of these three compounds
were obtained as shown in Figure 5.
20
ACS Paragon Plus Environment

Page 21 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
Figure 5. H-NMR spectra for (a)25DMR-h, (b)PG-a and (c)PG-anis in CDCl at room
3
temperature
The characteristic oxazine CH resonances are observed at 3.83 and 4.76 ppm for 25DMR-h,
2
4.43 and 5.27 ppm for PG-a, and 4.34 and 5.18 ppm for PG-anis, indicating that the intended
synthesis was successful.
The FT-IR spectra of those three fully substituted benzoxazine monomer and its heated samples
are shown in Figure 6. All three monomers, 25DMR-h, PG-a and PG-anis, show the
-1
characteristic band at 941, 948, and 936 cm , respectively, suggesting that this mode is not
21
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 22 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
exclusively the C-H out-of-plane bending mode of the benzene ring to which the oxazine ring is
attached, as historically assigned, since all three compounds lack the aromatic C-H group. The
possible contribution of C-H out-of-plane bending to this band, however, cannot be excluded by
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
o
this experiment. These bands completely disappear upon heat treatment at 180 C for 2 h,
strongly supporting that this characteristic mode relates to the oxazine group. Hence, it is now
possible to reassign this band as oxazine ring related. It is interesting to note that this
characteristic band for the all-substituted benzoxazine monomers is much shaper in comparison
to the monomers with C-H groups in the ring (for example Figure 8. Left. (a)). The absence of C-
H groups eliminates their contribution to the characteristic band. Thus, it is possible that this
band still correlates to the C-H out-of-plane bending vibration.
22
ACS Paragon Plus Environment

Page 23 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Comparison of FT-IR spectra of all-substituted benzoxazine monomer (top) and its
o
corresponding polymer obtained by heating the monomer at 180 C for 2 h (bottom): (a) 25DMR-
h and its polymer; (b) PG-a and its polymer; (c) PG-anis and its polymer.
In order to further verify the correctness of this new reassignment, various deuterated
benzoxazine monomers have been synthesized as follows.
3.1.2 Deuterium exchanged benzoxazine monomers.
Benzoxazines are often synthesized via one-pot Mannich condensation reaction by simply
mixing phenol, amine and formaldehyde. However, this method can only be used to synthesize
23
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 24 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
benzoxazines where no distinction of 2 and 4 positions of oxazine ring is required, if substitution
on the oxazine ring is desired. Thus, one-pot synthesis is used for the synthesis of PH-x, PH-x-
phenol-d , PH-x-oxazine-d and PH-d-a, as shown in Scheme 2 where x can be either a phenyl
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
or t-butyl group.
24
ACS Paragon Plus Environment

Page 25 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Scheme 2. Synthesis scheme of isotopes of PH-x, PH-x-phenol-d , PH-x-oxazine-d and PH-d-
4
4
a.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Synthesis scheme of isotopes of PH-x-2oxazine-d and PH-x-4oxazine-d .
2
2
In the case of PH-x-2oxazine-d and PH-x-4oxazine-d where the CH groups of the oxazine
2
2
2
group at the 2 and 4 positions, respectively, are selectively deuterated, multiple-step preparation
methods as shown in Scheme 3 were adopted. For the synthesis of PH-x-2oxazine-d monomer,
2
the amine was reacted with 2-hydroxybenzaldehyde, and the obtained compound was reduced by
sodium borohydride. Deuterated paraformaldehyde is used to close the ring, selectively
deuterating the CH group at the 2 position. Total yields of 78% and 48% for PH-a-oxazine-d2
2
and PH-t-oxazine-d were obtained, respectively. The lower yield for PH-t-oxazine-d is due to
2
2
the difficulty in precipitation of the intermediate 2-tbamp from water. The compound was
extracted from the aqueous layer. When the PH-a and PH-t series are compared, it is noted that
the PH-t series require less reaction time because of the higher basicity of alkyl chain amine than
25
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 26 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
aromatic amine. The reaction time decreases from 6 h to 2 h. Additionally, the PH-t series
eliminates completely the aromatic C-H signals that arise from the aromatic amine of the PH-a
series. For the case of PH-a-4oxazine-d monomer, the reaction starts from the PH-a-oxazine-
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
d4, opening the all deuterated oxazine ring by hydrolysis, then closing the ring by normal
paraformaldehyde.
In addition to the all-substituted benzoxazine monomers, the deuterium-exchanged benzene
structure will also help verify if the band in question is aromatic C-H origin as previously
-1
assigned. In case the prominent band in the range of 960-900 cm is suspected to arise from the
oxazine rather than the benzene, the deuterated oxazine ring in whole or in part will help identify
specific mode of the oxazine ring. The synthesis procedures adopted for each benzoxazine
monomer are shown in Scheme 2.
1
The successful synthesis of all these monomers with high purity are supported by the H NMR
spectra shown in Figure 7. The characteristic CH resonances of the oxazine ring are clearly
2
seen at 5.38 and 4.64 ppm for PH-a, 5.35 and 4.62 ppm for PH-a phenol-d and 5.39 and
4
4.66ppm for PH-d-a. These two singlets became only one resonance at 4.62 ppm for PH-a-
2oxazine-d , at 5.34 ppm for PH-a-4oxazine-d , and as expected, no CH -related resonances are
2
2
2
seen for PH-a-oxazine-d . Exactly the same trend is observed for PH-t series, also strongly
4
supporting that the expected synthesis had been successfully achieved.
26
ACS Paragon Plus Environment

Page 27 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
Figure 7. H-NMR spectra for PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a, (b) PH-
a-phenol-d , (c) PH-a-2oxazine-d , (d) PH-a-4oxazine-d , (e) PH-a-oxazine-d , (f) PH-d-a,
4
2
2
4
(g) PH-t, (h) PH-t-phenol-d , (i) PH-t-2oxazine-d , (j) PH-t-oxazine-d in CDCl at room
4 2 4 3
temperature.
The FT-IR spectra of all six compounds of the PH-a series and four of the PH-t series are shown
in Figure 8. The characteristic band in question did not significantly change its position for any
compound. Therefore, it is concluded that the band of interest should not be exclusively assigned
to either the aromatic C-H or the oxazine CH groups. It is interesting to note that the rather large
2
-1
-1
frequency shift, from around 940 cm to 915 cm , is observed when the substituent on the
27
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 28 of 74
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
oxazine ring changed from a heavy phenyl group to a lighter t-butyl group. Therefore, it is now
hypothesized that this mode in question may be of the oxazine skeletal mode.
-1
-1
Special attention should be paid to the band shift from around 753 cm to 582 cm in PH-t
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
series. The band can be assigned to the C-H out-of-plane bending of the aromatic group due to
4
4
the substitution of C-H by C-D.
Figure 8. FT-IR spectra of PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a, (b) PH-a-
phenol-d , (c) PH-a-2oxazine-d , (d) PH-a-4oxazine-d , (e) PH-a-oxazine-d , (f) PH-d-a, (g)
4
2
2
4
PH-t, (h) PH-t-phenol-d , (i) PH-t-2oxazine-d , (j) PH-t-oxazine-d .
4
2
4
1
.1.3. N-exchanged benzoxazine
5
3
28
ACS Paragon Plus Environment

Page 29 of 74
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Support that this characteristic mode is indeed the oxazine skeletal mode can also be obtained by
1
synthesizing N-exchanged monomer, PH-a- N. The synthetic procedure of PH-a- N is shown
5
15
15
in Scheme 4.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
15
Scheme 4. Synthesis scheme of isotopes of PH-a- N.
1
5
The FT-IR spectra of PH-a and PH-a- N are shown in Figure 9. The characteristic mode in
-1
15
question for PH-a is observed at 946 cm whereas that of the N isotope exchanged PH-a is at
-1
29 cm . The shift of the characteristic band from 946 cm to 929 cm demonstrates that N
-1
-1
15
9
isotope exchange influences the vibration significantly. The band can be either from direct
nitrogen atom participation or atoms that are very close to the nitrogen.
29
ACS Paragon Plus Environment