1H and 13C NMR detection of the carbocations or zwitterions from Rhodamine B base, a fluoran-based black color former, trityl benzoate, and methoxy-substituted trityl chlorides in the presence of alkali metal or alkaline earth metal

Article abstract of DOI:10.1246/bcsj.75.1569

The cleavage of C-O bonds in two fluoran-based dyes by the addition of alkali metal (M⁺ = Li⁺, Na⁺) and alkaline earth metal (M²⁺ = Mg²⁺, Ba²⁺) perchlorates in acetonitrile solution was exa

Full text of DOI:10.1246/bcsj.75.1569

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1569–1576 (2002) 1569
e Chemical
ciety of Ja-
n
1H and 13C NMR Detection of the Carbocations or Zwitterions from
Rhodamine B Base, a Fluoran-Based Black Color Former, Trityl
Benzoate, and Methoxy-Substituted Trityl Chlorides in the Presence of
Alkali Metal or Alkaline Earth Metal Perchlorates in Acetonitrile
Solution
e Chemical
ciety of Ja-
n
02
69
76
SJA8
09-2673
436
1H and 13C NMR Detection of the Carbocations
M. Hojo et al.
Masashi Hojo,* Tadaharu Ueda, Masanori Yamasaki, Akihiko Inoue, Sumio Tokita,† and
Mitsuhiro Yanagita††
Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520
01
†Department of Applied Chemistry, Faculty of Engineering, Saitama University, 255 Shimo-Ohkubo, Saitama 338-8570
††Research & Planning Department, Research & Technology Division, Nippon Soda Co., Ltd.,
2-1, 2-Chome, Ohtemachi, Chiyoda-ku, Tokyo 100-8165
02
.7
(Received December 7, 2001)
The cleavage of C–O bonds in two fluoran-based dyes by the addition of alkali metal (M+ = Li+, Na+) and alkaline
earth metal (M2+ = Mg2+, Ba2+) perchlorates in acetonitrile solution was examined by means of UV-visible absorption
1
1
and H and 13C NMR spectroscopy at room temperature. Although the H NMR signals of the xanthene part of
Rhodamine B base (3ꢀ,6ꢀ-bis(diethylamino)fluoran) were shifted toward lower fields, those of the isobenzofuran (or the
benzoate) part were not shifted much upon the addition of M+ and M2+. The appearance of 13C signals at 163.6 and
162.4 ppm (vs TMS) in the presence of Li+ and Ba2+, respectively, confirmed the formation of the zwitterions from
Rhodamine B base. The assignments of the H and 13C signals were performed by the HMBC method. The colored
1
zwitterion of a practical black color former, 2ꢀ-anilino-6ꢀ-dibutylamino-3ꢀ-methylspiro[3H-isobenzofuran-1, 9ꢀ-(9H)xan-
thene]-3-one, was produced by the addition of M+ and M2+. The 13C NMR signals at 162.2 and 159.8 ppm in the pres-
ence of 2.0 mol dm−3 Mg(ClO4)2 and Ba(ClO4)2, respectively, gave conclusive evidence of the formation of the sp2-hy-
brid carbon center for the black color former. The interaction between the metal ions and the zwitterions from the fluo-
ran-based dyes decreased as Mg2+ > Ba2+ > Li+ > Na+. The 1:1 complex formation constants with the metal ions for
both dyes were evaluated based on the UV-visible absorption data. The smaller formation constants of the black color
former, compared with those of Rhodamine B base, indicate some more difficulty in the cleavage of the γ-lactone ring,
which is probably based on the asymmetric structure of the compound, i.e, only a single dibutylamono-group at the 3ꢀ-
and 6ꢀ-positions. The evidence for the formation of both the trityl cation and the benzoate ion was obtained by the 1H
NMR signals from trityl benzoate in the presence of Mg(ClO4)2 in CD3CN containing a small amount of CF3SO3D. The
C–Cl bond cleavage of 4-methoxy, 4,4ꢀ-dimethoxy, and 4,4ꢀ,4ꢁ-trimethoxytrityl chlorides, through the chemical interac-
tion between Ba2+ and Cl− in acetonitrile soluition, was justified by our 13C NMR results.
Fluoran leuco dyes have been widely used as heat- and pres-
sure-sensitive recording or copying materials. The colors are
developed by the interaction between the color former and de-
velopers, such as active clay, phenols, and zinc salicylates.1
The application of other metallic ions (Al3+, Cd2+, Ca2+, and
Sr2+) was likewise supposed to improve the fade resistance and
give a similar intensity.2 The cleavage of the γ-lactone ring,
that is, the formation of zwitterions, is the cause of the deep
colors.3 Kortum and Vogel4 have reported that, on surfaces of
alkali halides and alkaline earth metal sulfates, the γ-lactone of
Malachite Green-o-carboxylic acid is cleaved to give a zwitte-
rion. They showed that the cleavage of the γ-lactone may be
caused by a polarizing effect of the cations on the surface of
the metal halide crystal lattice. Based on the 13C NMR, X-ray,
and crystallographic data, Rihs and Weis5 concluded the open
carboxylate structures for the complexes of Crystal Violet lac-
tone and a fluoran-derivative with Zn2+ or Cd2+, which were
prepared in acetone solution.
Hojo et al.6 have definitely demonstrated by means of elec-
trochemical and spectroscopic technics that alkali metal (M+)
and alkaline earth metal ion (M2+) can take part directly in
chemical reactions in solution. The formation of stable car-
bocations from 4-methoxy-substituted trityl chlorides ((4-
MeOC6H4)nPh3−nCCl), through the interaction between Cl−
1
and M+ or M2+, has been observed by H NMR.7 It may be
true that little attention has been paid to such “minor” interac-
tions. Gordon8 has stated that covalent bonding is believed to
be absent in alkali metal-halide and alkaline earth-fluoride ion
pairs. However, if carbocations are stabilized in the presence
of alkali metal and alkaline earth metal ions, the solvolysis re-

1570 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
1H and 13C NMR Detection of the Carbocations
Chart 1.
action rates in SN1 should be enhanced. This hypothesis has
been verified by exponential increases in the SN1 solvolysis
rates of aliphatic halides in the presence of concentrated metal
ions.9 Hojo et al.10 have proposed that “isolated” water mole-
cules (H–O–H), which are converted from bulk water by con-
centrated salts, may behave just like “dihydrogen ether”
([R]H–O–H[R]). The Raman spectra10,11 of concentrated salt
solutions have supported the idea of a “dihydrogen ether.” It
must be possible that a very small but direct “chemical” inter-
action between M+ or M2+ and simple anions, such as halide
and carboxylate ions, occurs even in aqueous or organic-aque-
ous solutions if they are in the “dihydrogen ether” conditions.
In kinetic studies, we have a merit to be able to amplify a mi-
nor increase in the concentration of an intermediate species,
which can never be detected in the equilibrium sense, into an
observable large increase in the reaction rate.
Chart 2.
metal and alkaline earth metal ions in solution and, secondly,
to propose alkaline earth metal perchlorates as new candidates
of color developers for the fluoran-based black color former in
an aprotic solvent. 1H NMR evidence is provided concerning
the complete dissociation or separation of the trityl cation from
trityl benzoate by the addition of Mg(ClO4)2 in the co-exist-
ence of a small amount of a “strong” acid. In this connection,
further studies on the formation of 4-methoxy-, 4,4ꢀ-
dimethoxy-, and 4,4ꢀ,4ꢁ-methoxytrityl cations from the trityl
chlorides in acetonitrile were performed using the 13C NMR
method.
Experimental
In a previous study,12 the cleavage of the γ-lactone ring of
Rhodamine B base and Crystal Violet lactone in the presence
of M+ or M2+ in acetonitrile was examined by means of UV-
visible spectroscopy. The color changes of sulfonephthaleins,
such as Phenol Red and Bromothymol Blue, were also attribut-
ed to cleavage of the γ-sultone rings through a direct interac-
tion between M+ or M2+ and the benzensulfonate ion (func-
tion) of the sulfonephthaleins in acetonitrile.13
Rhodamine B base (3ꢀ,6ꢀ-bis(diethylamino)fluoran) was pur-
chased from Aldrich. The preparation method of the fluoran-
based black color former, 2ꢀ-anilino-6ꢀ-dibutylamino-3ꢀ-methyl-
spiro[3H-isobenzofuran-1,9ꢀ-(9H)xanthene]-3-one, was described
in a previous paper.14 Trityl (TCI, GR grade), 4-methoxytrityl
(Aldrich, 97%), 4,4ꢀ-dimethoxytrityl (Aldrich, 95%), and 4,4ꢀ,4ꢁ-
trimethoxytrityl chlorides (Aldrich, tech. grade), and trityl ben-
zoate (Aldrich) were used without further purification. Methane-
sulfonic (Wako, GR grade) and trifluoromethanesulfonic acids
(Wako or TCI, GR grade), and CF3SO3D (Aldrich, 98 atom% D)
were used. Anhydrous salts of LiClO4 from Wako of GR grade,
NaClO4, Mg(ClO4)2, and Ba(ClO4)2 from Aldrich were used as re-
ceived. Acetonitrile, CH3CN (Wako, GR grade) and CD3CN (Ald-
rich, 99.6 atom% D) were used as UV-visible and NMR spectro-
scopic solvents, respectively.
1
In the present paper, we report on the H and 13C NMR
spectroscopic evidence of a γ-lactone cleavage of Rhodamine
B base (Chart 1) in the presence of alkali metal and alkaline
earth metal perchlorates in acetonitrile. The dark color devel-
opment from a practical fluoran-based black color former
(Chart 2) in the presence of the metal ions in acetonitrile solu-
tion was examined by means of UV-visible and 13C NMR
spectroscopy. Yanagita et al.14 have performed 13C NMR and
electronic absorption spectroscopic studies on the equilibrium
between the colorless lactone and the colored zwitterion forms
of the black color former in phenols. Several fluoran lueco
dyes were newly synthesized, and the color developments have
been observed not only in hydrogen-bond donor solvents, but
also in alkaline media for fluorans having an –NH-phenyl
group.15 The valence XPS and UV-visible absorption spectra
were analyzed using MO calculations for triphenylmethane
and fluoran dyes, including Crystal Violet lactone, Rhodamine
B base, and the black color former.16 The crystal and molecu-
lar structures of several fluoran-based color formers were de-
termined by single-crystal X-ray diffraction analysis.17
1H and 13C NMR measurements were carried out at room tem-
perature with JEOL FT-NMR spectrometers (Model JNM-LA400
and Model ECP500), and a Bruker AMX400. The signals were
assigned by two-dimentional (2D) NMR technics, such as HMBC
(heteronuclear multiple bond correlation) and others.14,15 UV-visi-
ble spectra were measured using a Hitachi double-beam spectro-
phtometer (Model U-2000) in a 1.0 cm quartz cuvette at room
temperature.
Results and Discussion
Effects of Alkali Metal and Alkaline Earth Metal Ions on
the -Lactone Ring-Opening of Rhodamine B Base and a
γ
1
Black Color Former. Figure 1 shows the H NMR spectral
The main purposes of the present study were, firstly, a fur-
ther confirmation of the direct chemical interaction with alkali
changes of 3.0 × 10−3 mol dm−3 Rhodamine B base (cf. Chart
1) with increasing concentration of LiClO4 in CD3CN at room

M. Hojo et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1571
Fig. 2. Changes in the chemical shift of Rhodamine B base
(at the 4-position, cf. Chart 1) in the presence of metal per-
chlorates in CD3CN: (ꢀ) LiClO4; (ꢁ) NaClO4; (ꢂ)
Ba(ClO4)2; (ꢃ) Et4NClO4.
The color changes of Rhodamine B base from no color to a
deep pink-red upon the addition of alkali metal (M+ = Li+,
Na+) and alkaline earth metal ions (M2+ = Mg2+, Ba2+) had
been examined by UV-visible absorption spectroscopy at a
lower concentration of Rodamine B base (2.0 × 10−4 mol
dm−3).12 The color change has been attributed to the γ-lactone
opening through a direct interaction between the metal ions
and the benzoate ion (function) in acetonitrle solution. The
formation constants between the Rhodamine B base zwitterion
(RB ) and the metal ions were evaluated using previous data:12
log K = 3.0, 1.9, and 0.5 for Ba2+, Li+, and Na+, respectively,
assuming 1:1 interaction, as follows:
1
Fig. 1. The H NMR spectra of 3.0 × 10−3 mol dm−3
Rhodamine B base with increasing concentration of
LiClO4 in CD3CN at room temperature: (a) 0; (b) 0.01; (c)
0.1; (d) 0.2; (e) 0.5; (f) 1.0 mol dm−3 LiClO4.
temperature. The proton signals on the xanthene part in
Rhodamine B base shifted to lower magnetic fields, while
those on the isobenzofuran (or benzoate) part remained at the
original value, except for that at the 12-position. Upon the ad-
dition of only 0.01 mol dm−3 LiClO4, large spectral shifts were
observed. With 0.5 mol dm−3 LiClO4 and more, the chemical
shifts reached constant values, e.g., δ = 6.84 for the 4-position
of Rhodamine B base (cf. Fig. 2). The effects of NaClO4 were
much smaller than those of LiClO4, although the δ value at the
4-position approached the maximum value at 1.0 mol dm−3
NaClO4. Barium perchlorate, even at a much lower concentra-
tion, 0.01 mol dm−3, caused almost the ultimate shift. A non-
RB + M+ (or M2+) ꢀ RB M+ (or M2+).
(1)
For evaluating the formation constants, the absorbances around
545 nm in the presence of different concentrations of the metal
ions were utilized; the molar absorptivity of the RB M+ (or
M2+) was ε = 1.1 × 105 cm−1 mol−1 dm3. The analytical
method described in a previous paper20 was applied with some
modifications. The interaction between RB and Mg2+ was
too large to evaluate the formation constant with > 1 × 10−4
mol dm−3 Mg(ClO4)2; the formation of Mg2+(RB )2 was sus-
pected in the presence of lower Mg2+ concentrations.
1
1
metallic salt, Et4NClO4, caused almost no effects on the H
The shifts in the δ values of H NMR toward lower fields
NMR signals. However, the spectrum of Rhodamine B base
was deformed upon the addition of Mg(ClO4)2 in CD3CN, al-
though many signals shifted toward low fields with increasing
concentration of the metal perchlorate. The addition of an ac-
id, CF3SO3D, also caused complexity in the spectrum. Hydro-
gen ions (H+ and D+) and Mg2+ could interact not only with
the carboxylate-O atom (COO−), but also with the diethylami-
no-N atom (Et2N-) of Rhodamine B base. The poly-protonated
species, RH22+ and RH33+, as well as RH+ of Rhodamine B in
an acidic aqueous solution were observed.18 Hojo et al.19 have
reported a strong interaction between Mg2+ and N-atoms of
“Methyl Yellow,” p-(dimethylamino)azobenzene, in acetoni-
trile.
with increasing concentrations of the metal ions are in good
accordance with the increase in the color intensity12 from
Rhodamine B base. We have demonstrated that the formation
of the trityl cations in the presence of M+ or M2+ causes re-
markable low field shifts in the δ value of 1H NMR.7 The for-
mation of the zwitterion or the carbocation center from
Rhodamine B base was strongly suggested by the shifts in the
δ value with increasing the concentration of M+ or M2+ in ace-
tonitrile. The following 13C NMR data directly demonstrates
the opening of the γ-lactone ring of Rhodamine B.
Figure 3 shows the 13C NMR spectra of Rhodamine B base
in the presence of LiClO4 in CD3CN. In the absence of metal
ions, the signal of C-7 combined with the carboxylate O-atom

1572 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
1H and 13C NMR Detection of the Carbocations
Fig. 3. The 13C NMR spectra of 0.01 mol dm−3 Rhodamine
B base in CD3CN in the presence of LiClO4: (a) 0; (b) 0.1;
(c) 1.0 mol dm−3 LiClO4.
Fig. 4. Effects of Mg(ClO4)2 on the UV-visible absorption
spectrum (path-length: 1.0 cm) of the black color former
(5.0 × 10−5 mol dm−3) in CH3CN; (———) 0; (- - - - -)
5.0 × 10−5; (– – –) 1.0 × 10−4; (— - —) 1.0 × 10−3
;
Table 1. The Chemical Shift Valuesa) of 13C NMR Signals of
Rhodamine B Base in the Presence of Ba(ClO4)2 in CD3CN
at Room Temperature
(— - - —) 0.01; (———) 0.1 mol dm−3 Mg(ClO4)2.
Carbon no.
δ b)
δ c)
Carbon no.
δ b)
δ c)
1
2
3
4
5
6
7
8
132.2 128.8
114.3 108.3
155.8 149.7
9
10
11
12
13
14
15
16
129.8 123.9
131.1 135.0
130.6 129.6
130.0 124.5
133.1 127.5
174.3 169.4
95.9
97.2
158.2 153.2d)
114.0 105.7
162.4
85.2
45.9
12.3
44.2
11.8
138.2 153.2d)
a) Vs TMS. b) In the presence of 1.0 mol dm−3 Ba(ClO4)2
for 0.04 mol dm−3 Rhodamine B base. c) In the absence of
the salt. d) The both signals at nos. 5 and 8 positions were
overlapped each other. Also in CDCl3, the signals were
found to be overlapped (153.26 and 153.38 ppm).
Fig. 5. Changes in absorbance at λmax (ca. 570–585 nm) of
5.0 × 10−5 mol dm−3 black color former with increasing
concentration of various salts and acids in CH3CN: (ꢀ)
LiClO4; (ꢁ) NaClO4; (ꢄ) Mg(ClO4)2; (ꢅ) Ba(ClO4)2; (ꢃ)
CH3SO3H; (ꢆ) CF3SO3H.
appeared at 85.2 ppm (vs TMS). However, the signal shifted
to 163.6 ppm (∆δ = +78.4) in the presence of 1.0 mol dm−3
LiClO4. Even at 0.1 mol dm−3 LiClO4, a new peak at around
163 ppm of C-7 could be observed. The great low-field shift in
the 13C NMR signal of C-7 indicates the formation of the car-
bocation (sp2) center, or the opening of the γ-lactone ring. In
the presence of 1.0 mol dm−3 Ba(ClO4)2, the C-7 signal ap-
peared at 162.4 ppm (∆δ = +77.2); the chemical-shift values
of signals in the absence and presence of Ba(ClO4)2 are listed
in Table 1. We would like to mention that no precipitation was
observed in a mixed solution of Rhodamine B base and metal
perchlorates.
The γ-lactone ring of a fluoran-based black color former
(Chart 2) was also cleaved by the alkali metal and alkaline
earth metal ions in an acetonitrile solution. Figure 4 shows the
effects of Mg(ClO4)2 on the UV-visible absorption spectra of
the black color former (5.0 × 10−5 mol dm−3) in CH3CN at
room temperature. Without the Mg2+ ion, the black color
former gave no absorption at > 370 nm. Upon the addition on
an equivalent amount of Mg2+, the no color solution turned to
be a dark color. The color intensity increased with increasing
concentration of Mg2+, and reached its maximum value at 0.1
mol dm−3 Mg2+. The absorbances at λmax were 1.34, 0.91, and
1.05 at 310, 450, and 576 nm, respectively. The absorptivity of
2.1 × 104 at 576 nm was larger than any value observed in a
variety of acidic media.14 An isosbestic point was given at 293
nm in CH3CN. The effects of Ba2+ were smaller than those of
Mg2+; at 1.0 mol dm−3 Ba(ClO4)2, the peak absorbances
seemed to reach barely their maximum values: abs = 1.25,
0.86, and 0.97 at 305, around 453, and 570 nm. The isosbestic
point was also observed at 291 nm for Ba2+. The effects of al-
kali metal ions were definitely smaller than those of the alka-
line earth metal ions (cf. Fig. 5). The 1:1 formation constants
between the metal ions and the zwitterion from the black color
former in CH3CN were evaluated as follows: log K = 3.3, 0.4,
and −0.5 for Mg2+, Ba2+, and Li+, respectively. As described

M. Hojo et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1573
above, much larger formation constants with the correspond-
ing metal ions were given for Rhodamine B base. The asym-
metric structure, i.e., only a single dialkylamino-group at 3-
and 6-positions of the black color former (cf. Charts 1 and 2),
should cause a difficulty in cleaving the γ-lactone ring.
The effects of rather strong acids on the black color former
in CH3CN were also examined. Upon the addition of 1.0 ×
10−4 mol dm−3 methanesulfonic acid to 5.0 × 10−5 mol dm−3
of the color former, the peak intensities reached their maxima:
abs = 1.29, 0.88, and 1.04 at 309, around 448, and 585 nm,
displaying an isosbestic point at ca. 293 nm. The absorption
spectra were almost identical in the presence of a range of 1.0
× 10−3–0.01 mol dm−3 methanesulfonic acid. However, in the
presence of 0.1 mol dm−3 and more of the acid, the absorbanc-
es of the two peaks decreased; at 0.5 mol dm−3, the peaks dis-
appeared to be shoulders and, instead, a new band appeared
between the (previous) two peaks; the new band consisted of
three peaks at 467, 497, and 535 nm. Further, a stronger acid,
trifluoromethanesulfonic acid, CF3SO3H, caused the black col-
or to turn into a red color with much lower concentrations of
the acid. At the 1.0 × 10−2 mol dm−3 acid, the peak of the
black color species disappeared, and the absorbances of the
new triple peaks of the red color species approached their max-
imum values: abs = 1.05, 1.32, and 0.97 at 470, 496, and 530
nm, respectively. Remarkable isosbestic points were given at
455 and 543 nm in the presence of 5.0 × 10−5–0.01 mol dm−3
acid.
Fig. 6. The 13C NMR spectra of the black color former (0.04
mol dm−3, cf. Chart 2) in the absence and the presence of
Ba(ClO4)2 in CD3CN: (a) no salt; (b) in the presence of 2.0
mol dm−3 Ba(ClO4)2.
The red color should have been caused by monoprotonation
to the anilino-N group of the black color species, although the
dialkylamino-N is a stronger base than the anilino-N in the
usual case. A MO calculation showed that the electron density
on N-atom at the 2-position (−0.200) is smaller than that at the
6-position (−0.243) for the zwitterion form, whereas the oppo-
site situation holds for the original lactone form. Electronic
spectrum studies of the derivative compounds also support our
speculation. Monoprotonation to the dialkylamino group of
Rhodamine B in acetonitrile caused a red color species: an ab-
sorption band with three peaks at ca. 460, 493 nm, and ca. 530
nm. The acidities of CH3SO3H and CF3SO3H are not very
strong in an aprotic solvent, acetonitrile, because of its poor
solvation ability;21 the pKa values were reported to be 8.36 and
2.60 for CH3SO3H and CF3SO3H, respectively.22,23
definite signal appeared at 162.2 ppm in the presence of 1.0
mol dm−3 Mg(ClO4)2; however, the signal of C-20 became
rather obscure in this case.
Yanagita et al.14 have observed the 13C signal of C-9 of the
black color former at 162.7 ppm in a normal color developer,
phenol-d6. However, upon increasing the temperature (from
40 to 140 °C), i.e., with the incompleted cleavage of the γ-lac-
tone ring, the 13C NMR signal of the carbocation center shifted
remarkably toward higher fields, while the signal of C-14 shift-
ed toward a lower field. Similar phenomena have also been re-
ported for a chloro-derivative of a black color former, 2ꢀ-(o-
chlroanilino)-6ꢀ-dibutylamino-3ꢀ-methylfluoran, in phenol-
d6.24 Therefore, the peaks at slightly higher magnetic fields
than δ = 162 ppm should be caused mainly by an incomplete
cleavage of the γ-lactone ring. Another reason may account for
the variable values in the chemical shifts for the ring-opened
C-9: the stronger interaction between the metal ions and the
carboxylate ion (function) within the fluoran compound would
cause a higher field shift. In the 13C signal of C-20, also, the
metal cation–O− interaction may cause different chemical
shifts: 170.7 and 173.2 ppm for 1.0 mol dm−3 Mg(ClO4)2 and
Ba(ClO4)2, respectively. Similar arguments also seem to be
valid for the effects of Li+ and Ba2+ on Rhodamine B base.
Trityl Benzoate and Methoxy-Substituted Trityl Chlo-
rides. A direct interaction between the metal ions and the
By the way, the addition of 1 × 10−3 mol dm−3 Et4NOH or
Me4NOH to the black color former of 5.0 × 10−5 mol dm−3 in
CH3CN caused the solution to become pale yellow, giving a
broad peak at around 420 nm, although the yellow solution re-
turned to almost colorless in the presence of > 1 × 10−2 mol
dm−3 of the strong alkalis. The color development should have
been caused by the anion which is produced by deprotonation
of the –NH group.15 A weaker base, 1,1,3,3-tetramethylguani-
dine, at least, up to 0.1 mol dm−3, caused no change in the ab-
sorption spectrum of the black color former.
Figure 6 shows the 13C NMR spectra of a 0.04 mol dm−3
black color former in the presence of Ba(ClO4)2 in CD3CN.
The signal at 83.9 ppm of C-9 was shifted to 159.8 ppm in the
presence of 2.0 mol dm−3 Ba(ClO4)2. A broad signal appeared
at a slightly higher field, ca. 157.5 ppm, with 1.0 mol dm−3
Ba(ClO4)2. In the presence of 2.0 mol dm−3 LiClO4, an incom-
plete peak seemed to be observed at ca. 158.5 ppm, whereas a
1
benzoate ion was examined by H NMR spectroscopy. We
have demonsrtrated that the alkali metal ions in acetonitrile
could have direct interactions with benzoate ions by means of
polarography25 and UV-visible absorption spectroscopy.20 The
C–O bond of trityl benzoate could never be cleaved by the ad-
dition of just M+ or M2+ in acetonitrile. However, in the co-

1574 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
1H and 13C NMR Detection of the Carbocations
benzoate with the assistance of a small amount of a “strong”
acid in acetonitrile. The mechanism for the cleavage of the C–
O bond of trityl benzoate in the co-existence of H+ and Mg2+
has been provided in the previous paper.12
In this connection, the formation of trityl ions from meth-
oxy-trityl chlorides, based on the interaction between Ba2+ and
Cl− in acetonitrile, was examined by 13C NMR. In the previ-
ous paper, Hojo et al.7 have reported that the 1H NMR signals
of 4-methoxytrityl chlorides were greatly shifted toward lower
fields by the addition of the perchlorate salts of alkali metal
(Li+, Na+) and alkaline earth metal ions (Mg2+, Ca2+, Sr2+,
Ba2+). It has been found that barium perchlorate is the most
effective salt among them because of the precipitation of the
mixed anion salt, BaCl(ClO4), from acetonitrile solution,26 as
follows where n = 1–3:
(4-MeOC6H4)nPh3−nCCl + Ba(ClO4)2
−
→ (4-MeOC6H4)nPh3−nC+ClO4
+ BaCl(ClO4) ↓ (precipitation).
(2)
The above reaction has been utilized for a new preparation
method of methoxy-substituted trityl perchlorates from the
corresponding chlorides in acetonitrile,7 instead of using per-
chloric acid or AgClO4.27
Figure 8 shows the 13C NMR spectra (of the supernatant so-
lution) of 4-methoxytrityl, 4,4ꢀ-dimethoxytrityl, and 4,4ꢀ,4ꢁ-tri-
methoxytrityl chlorides in the presence of 1.0 mol dm−3
Ba(ClO4)2 in CD3CN at room temperature. The signals around
200 ppm indicate the formation of the methoxy-substituted tri-
tyl cations: 200.8, 196.9, and 194.0 ppm for 4-methoxytrityl,
4,4ꢀ-dimethoxytrityl, and 4,4ꢀ,4ꢁ-trimethoxytrityl ions, respec-
tively. These values seem to be quite reasonable, based on the
values of unsubstituted-trityl28 and 4,4ꢀ,4ꢁ-trimethoxytrityl
ions29 (δ = 212 and 193.2 ppm, respectively) in the conc.
H2SO4 solvent. The assignments of other carbon atoms were
performed, following the literature data.28,29 The chemical
shift values of the signals are listed in Table 2. As for unsubsti-
tuted-trityl chloride, in the presence of the alkali metal and al-
kaline earth metal ions in acetonitrile solution, the trityl ion
was not able to be observed by the NMR method, while the
UV-visible absorption spectra12 gave evidence of the formation
of the trityl ion (ca. 25 and 40% yields, respectively) from 1.0
× 10−3 mol dm−3 trityl chloride and bromide in the presence
of 1.0 mol dm−3 Ba(ClO4)2. UV-visible spectroscopy makes it
possible to observe the formation of the trityl ion, even at low-
er concentrations, because of its large absorptivity of 4.0 × 104
cm−1 mol−1 dm3 at 403 nm. However, a strong interaction be-
tween R3C+ and the mother compound, R3CX, causes line-
broadening in NMR signals, which must make it difficult to
observe the formation of the trityl ions at lower yields (<
50%), even if the absolute concentration of the trityl ion is high
enough for the NMR observation. As for the stabilities of the
carbocations, a methoxy group brings great stability to the tri-
tyl ion.7,12,30
Fig. 7. Changes of 1H NMR spectra of 2.7 × 10−3 mol dm−3
trityl benzoate with increasing concentration of Mg(ClO4)2
in the presence of 2.7 × 10−3 mol dm−3 CF3SO3D in
CD3CN: (a) no acid; (b) no metal ion; (c) 0.01; (d) 0.1; (e)
0.5; (f) 1.0 mol dm−3 Mg(ClO4)2; (g) 0.5 mol dm−3
CF3SO3D. The bold signals show the positions on the tri-
tyl ion, and the others on the benzoate.
existence of a small amount of CF3SO3D, it has been found by
means of UV-visible absorption spectroscopy that the magne-
sium ion has the ability to cleave the C–O bond to produce the
trityl ion.12 In the present study, the formation of not only the
trityl ion, but also the benzoate ion from trityl benzoate, was
observed by 1H NMR, as follows.
Trityl benzoate in CD3CN gave signals of the trityl com-
pound (at around 7.3 ppm) and small signals of benzoic acid in
the absence of strong acids or metal ions; the signals of benzo-
ic acid developed upon the addition of an equivalence amount
of CF3SO3D, but still almost no signals of the trityl ion ap-
peared (cf. Fig. 7). However, by the addition of 0.01 mol
1
dm−3 Mg(ClO4)2, broad, but definite, H NMR signals of the
trityl ions were produced. At 0.1 mol dm−3 Mg(ClO4)2, the
signals of Ph3C+ developed further at the expense of those of
trityl benzoate (Ph3CX). Strange to say, however, with 0.5 mol
dm−3 Mg(ClO4)2, the signals of a trityl compound appeared
again, although the signals of Ph3C+ were displayed more
sharply. Here, the formation of Ph3COH, based on the reaction
between Ph3C+ and H2O introduced with added Mg(ClO4)2
(hydrated water as the impurity), may be suspected. It goes
without saying that, in the presence of 0.5 mol dm−3 CF3SO3D,
trityl benzoate was converted into the trityl ion and benzoic ac-
The present study demonstrates that a strong interaction can
operate between the metal ions (alkali metal and alkaline earth
metal ions) and anions to produce stable carbocations or zwit-
terions in solution when ions are not strongly solvated. We be-
lieve that some features in concentrated salt effects on SN1-
1
id, completely. The H NMR results, mentioned above, con-
firm that the magnesium ion can cleave the C–O bond of trityl

M. Hojo et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1575
Fig. 8. Effects of 1.0 mol dm−3 Ba(ClO4)2 on the 13C NMR spectra of 0.01 mol dm−3 trityl chlorides in CD3CN: (a) trityl chloride
with 0.5 mol dm−3 CF3SO3H and no salt; (b) 4-methoxytrityl chloride; (c) 4,4ꢀ-dimethoxytrityl chloride; (d) 4,4ꢀ,4ꢁ-trimethoxytri-
tyl chloride. The primed numbers represent the positions on the methyoxy-substituted phenyl groups.
Table 2. The Chemical Shift Valuesa) of 13C NMR Signals of Trityl Chlorides and the Trityl Cations in CD3CN at Room Tempera-
ture
C-0
C-1
C-1ꢀb)
C-2
C-2ꢀb)
C-3
C-3ꢀb)
C-4
C-4ꢀb)
Ph3CCl
82.8
213.1
200.8
196.9
194.0
146.2
141.3
140.3
140.1
—
—
—
134.5
133.6
133.1
130.5
144.1
140.2
139.2
—
—
—
148.8
145.6
143.9
129.0
131.2
130.3
130.1
—
—
—
119.2
117.7
117.0
128.1
144.1
139.7
138.5
—
—
—
177.1
173.0
171.1
Ph3C+ c)
(4-MeOC6H4)Ph2C+ d)
(4-MeOC6H4)2PhC+ d)
(4-MeOC6H4)3C+ d)
a) Vs TMS. b) The primed numbers represent the positions on the methoxy-substituted pheny groups. c) In the presence of 0.5
mol dm−3 CF3SO3H for 0.01 mol dm−3 trityl chloride. d) In the presence of 1.0 mol dm−3 Ba(ClO4)2 for 0.01 mol dm−3 trityl
chlorides.
6
M. Hojo, T. Takiguchi, M. Hagiwara, H. Nagai, and Y.
type solvolysis rates for RX (organic halides) are certainly
caused by direct interactions between the X− ion and the metal
ions (M+, M2+) in aqueous organic solutions; in these solu-
tions, the water structure through hydrogen bonging is distort-
ed by organic solvents as well as by concentrated salts. Raman
spectroscopic evidence for the increased distortion of water
structure (i.e., the conversion of bulk water into “dihydrogen
ether”)10,11 by the addition of organic solvents, such as acetoni-
trile, will be reported later.
Imai, J. Phys. Chem., 93, 955 (1989); M. Hojo, Y. Miyauchi, A.
Tanio, and Y. Imai, J. Chem. Soc., Faraday Trans., 87, 3847
(1991); Y. Miyauchi, M. Hojo, H. Moriyama, and Y. Imai, J.
Chem. Soc., Faraday Trans., 88, 3175 (1992); M. Hojo, Y.
Miyauchi, N. Ide, A. Tanio, and Y. Imai, J. Electroanal. Chem.,
340, 197 (1992); M. Hojo, H. Hasegawa, Y. Miyauchi, H.
Moriyama, H. Yoneda, and S. Arisawa, Electrochim. Acta, 39, 629
(1994); M. Hojo, H. Hasegawa, and H. Yoneda, J. Chem. Soc.,
Perkin Trans. 2, 1994, 1855; M. Hojo, H. Hasegawa, A. Mizobe,
Y. Ohkawa, and Y. Miimi, J. Phys. Chem., 99, 16609 (1995); M.
Hojo, H. Hasegawa, and N. Hiura, J. Phys. Chem., 100, 891
(1996); Z. Chen and M. Hojo, J. Phys. Chem. B, 101, 10896
(1997); M. Hojo, T. Ueda, M. Nishimura, H. Hamada, M. Matsui,
and S. Umetani, J. Phys. Chem. B, 103, 8965 (1999).
The present study was partially supported by the JAERI’s
Nuclear Research Promotion Program, Japan.
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