Exam 1 Flashcards
(106 cards)
1
Q
Spectroscopy
A
The measurement of light (or electromagnetic radiation)
2
Q
λ
A
Wavelength
One complete wave cycle
3
Q
ν
A
Frequency
The number of cycles per time period
ν = c / λ
4
Q
ν ~
A
Wavenumber
ν ~ = 1 / λ
5
Q
E
A
Energy
E = hν = hc / λ
6
Q
UV, X Rays, Gamma Rays +
A
Ionization
7
Q
Visible, near UV
A
Electronic Transitions
8
Q
Infrared (IR)
A
Molecular Bond Vibration
** IR Spectroscopy **
9
Q
Microwave
A
Rotational Molecule Motion
10
Q
Radio
A
Nuclear Spin Transitions
** NMR Spectroscopy **
11
Q
What is the function of IR Spectroscopy?
A
To identify functional groups on an unknown molecule by looking at differences in molecular vibrations.
12
Q
What determines the X and Y position s of peaks in IR spec?
A
X: Determined by Beer’s Law ( A = ɛ c l) and Bond Polarity (more polar bonds are more deshielded = higher ppm) that change the frequency
Y: Determined by the mass of atoms in the bond (heavier atoms = higher ppm) and the bond strength (stronger bonds are more shielded = lower ppm)
13
Q
Affect of Resonance on IR Spec Peaks
A
Broaden peaks
Lowers frequency = higher ppm
14
Q
Affect of Hydrogen Bonding on IR Spec Peaks
A
Broadens peaks
(ex: O-H peak)
15
Q
What happens with symmetrical molecules in IR Spectrums?
A
Not seen because a dipole moment is needed to see a peak in IR
16
Q
Shielding
A
More electron density = less interaction with magnetic field = lower ppm = upfield = more shielded
Less electron density = more interaction with magnetic field = higher ppm = downfield = less shielded (deshielded)
17
Q
What is required for a bond to be IR active?
A
- Applied radiation E needs to match the energy of a bond stretching or bending.
- Needs to result in a change in molecular polarity.
18
Q
What is Beer’s Law?
A
A = ϵ c l
A = abosrption (intensity of peak on Y-axis)
ϵ = molar absorptivity constant (change in polarity)
c = concentration
l = path length (thicker sample = more absorption)
19
Q
IR Template
A
20
Q
Ester / Ether
C - O Stretch
A
1200 - 100
Strong peaks
21
Q
Aromatic Ring
C = C Stretch
A
1700 - 1500
Variable peak with Overtones to the Left
22
Q
Alkene
C = C Stretch
A
1680 - 1620
Variable Peaks
23
Q
Ester
C = O Stretch
A
1750 - 1735 cm-1
~1735 cm -1
Strong Peak
24
Q
Aldehyde
C = O Stretch
A
1740 - 1690 cm-1
~1725 cm-1
Strong Peak
25
Ketone
C = O Stretch
1750 - 1680 cm-1
~1710 cm-1
Strong Peak
26
Carboxlyic Acid
C = O Stretch
1780 - 1710 cm-1
~1710 cm-1
Strong Peak
27
Amide
C = O Stretch
1690 - 1630 cm-1
~1650 cm-1
Strong Peak
28
Alkyne
R - C ≡ C Stretch
2260 - 2100
Variable Stretch
29
Nitrile
R - C ≡ N Stretch
2260 - 2220
Medium Peak
30
Alkyl sp3
C - H Stretch
2950 - 2850
Jagged Strong Peaks
31
Alken sp2
C - H Stretch
3100 - 3010
Medium Peaks
32
Carboxlyic Acid
O - H Stretch
Centered ~3000
Broad, Strong/Variable Peak
33
Alcohol
O - H Stretch
3550 - 3200
Very Broad, Strong Peak
34
Amine
R - N - H Stretch
3500 - 3300
Medium Peak
\*\* One Peak w/One Terminal H, Two Peaks w/Two Terminal H's \*\*
35
Amide
N - H Stretch
3700 - 3500
Medium Peaks
36
Alkyne
R - C ≡ H Stretch
~3300
Variable Peak
37
What information is provided by mass spectrometry?
1. Molecular weight of compound
2. High resolution MS = gives molecular formula
3. Some bond connectivity information (mroe from 'H NMR)
38
What is being measured in MS?
The mass of ions (generally cations)
39
Cation Generation
Electron Impact (EI) Ionization
\*\* Creates the radical cation represented in M+ Peak \*\*
40
Easiest Electrons to Ionize
1. Lone Pair electrons
2. Pi electrons (triple/double bonds)
3. Sigma electrons (single bonds)
41
What does Fragmentation Depend On?
1. Carbocation Stability: the compound will fragment at the spot that creates the most stable C+ firstmost
\*\* Substitution, Induction Effect, and Hyperconjugation affect C+ stability \*\*
2. Radical Stability: when mutliple stable C+ are possible, then the more stable radical will be made secondarily
42
Carbocation Stability
43
Why is MS often Coupled with GC?
GCMS provides rapid indication of a reaction
(GC = separation into individual components)
44
What is the molecular ion peak?
The peak furthest to the right that correlates to the molecular weight of your compound
\*\* The unfragmented radical cation \*\*
45
What is the base peak?
The largest peak in the spectrum that represents the most stable carbocation
46
Alcohol fragmentation
1. Alpha fragmentation (forming a carbonyl)
2. Kick off β-Hydrogen (protons shift to create water that will leave)
47
What is the difference between Low Resolution (LR) and High Resolution (HR) mass spectrometry?
LR: nominal (integer) masses
HR: Masses go to the fourth decimal place, provides the molecular formula
48
Br Isotope Pattern
| (Mass Spec)
M+ ≈ M+2
49
Cl Isotope Pattern
| (Mass Spec)
M+2 ≈ 1/3 M+
50
I Isotope Pattern
| (Mass Spec)
Peak at 127
M+ 127 away from next peak
51
S Isotope Pattern
| (Mass Spec)
M+2 \> M+1
\*\* Very subtle because both peaks are VERY small \*\*
52
N Isotope Pattern
| (Mass Spec)
A compound with an odd number of Nitrogens with have an odd nominal mass, so M+ will be on an odd number
53
What information is provided by 'H NMR?
1. The number of hydrogen environments in a structure (the number of peaks)
2. The number of hydrogens in each environment (integration of area under the peaks)
3. Functionality in compound (chemical shift)
\*\* 4. Provides connectivity information (spin-spin splitting - how many hydrogens are interacting)
5. Provides stereochemistry information (coupling constants differentiate between cis and trans)
54
What information is provided by 13C NMR?
1. The number of carbon environments (the number of peaks)
2. Functionality in compound (chemical shift)
3. The number of hydrogens attached to each carbon (\*\* when using off-resonance or DEPT 13C NMR \*\*)
55
Which nuclei have a nuclear spin?
1. Odd number of protons (odd atomic number)
OR
2. Odd number of protons and neutrons (odd mass number)
56
How can you increase NMR peak resolution?
You can increase the radio frequency or teh external magnetic field to increase the amount of nuclei with an α spin (lower energy, bigger gap = more resolution).
57
What challenges delayed the development of 13C NMR?
1. Isotope Abundance: there's only 1/100 carbons measurable (H: 100% abundance 1H; C: 99% abundance 12C and 1% abundance 13C)
2. Low gyromagentic ratio = low energy gap = low NMR resolution (γ 13C = 1/4 γ 1H using ΔE = γ (h/2π) B0)
58
What are the method for acquiring 13C NMR data?
1. Continuous Wave (CW): fixed magnetic field and nuclei that are gradually scanned from low frequency to high frequency (\*\* slow \*\*)
2. Foureier Transform (FT): a pulse of radiation that excites all the nuclei at once, causing them to precess. Then the relaxation to ground state is measured (by a computer program), creaing a FID that's translated into meaningful data (\*\* fast \*\*)
59
What is spin-spin splitting?
Protons within 2-3 bonds of each other can effect the chemical shifts of the neighboring environments
Ex: If Ha has an α spin, the chemical shift of Hb is slightly increased. If Ha has a β spin, the chemical shift of Hb is slightly decreased - creating 1:2:1 ratio (triplet peak)
60
What do EWG and EDG do to chemical shifts?
EWG take electron density away from the protons, deshielding them more than usual, so their chemical shifts move upfield (gets bigger)
\*\*Ex: Aromatic rings with EWG move from one peak at ~7.5 to two peaks between 8 - 7.5
EDG give electron density to the protons, shielding them more than usual, so their chemical shifts move downfield (gets smaller)
\*\*Ex: Aromatic rings with EDG move from one peak at ~7.5 to two peaks between 6-7.5
\*\* The effects of these groups drop off quickly with distance = strongly changes α hydrogens and softly changes β hydrogens \*\*
61
CH3 Alkane Chemical Shift
| (NMR)
0.9
| (~1)
62
CH2 Alkane Chemical Shift
| (NMR)
1.3
| (0.5 - 1.5)
63
CH Alkane Chemical Shift
| (NMR)
1.4
| (1 - 1.9)
64
Methyl Ketone + Any Protons α to a Carbonyl
(NMR)
2.1
| (1.5 - 2.5)
65
C ≡ C - H (Hydrogen Bond) Chemical Shift
(NMR)
2.5
| (2 - 3)
66
Aldehyde Chemical Shift
| (NMR)
9-10
67
Ester (ROOCR) Chemical Shift
| (NMR)
2 - 2.5
68
R-CH2-X (X = halogen) Chemical Shift
| (NMR)
3 - 4
69
Ether (RO-CH2-R) Chemical Shift
| (NMR)
3 - 3.5
70
ROOC-CH2-R Ester Chemical Shift
| (NMR)
3 - 3.5
71
Vinyl C = C - H Chemical Shift
| (NMR)
5 - 6
\*\* Vinyl = directly next to double bond, on double bonded carbon \*\*
72
Allylic C = C - CH3 Chemical Shift
| (NMR)
1.7
\*\* Allylic = on carbon bonded to the double bond carbon, one bond away \*\*
73
Aromatic Ph - H Chemical Shift
| (NMR)
7 - 8
| (~7.5)
74
Benzylic Ph - CH3 Chemical Shift
| (NMR)
2 - 2.5
| (~2.3)
75
Carboxylic Acid (HOOCR) Chemical Shift
| (NMR)
10 - 12
76
Why do esters and vinyl hydrogens absorb further downfield?
Their resonance structures place carbocations on carbons containing the hydrogen you inspect, which strongly deshields it = more downfield.
77
Why do hydrogens bound to arenes and alkenes absorbe further downfield than alkoxy groups?
They have an induced field created by their pi bonds that strengthen the external magnetic field, creating larger chemical shifts as the energy gap between α and β spinning nuclei increases.
78
Which energy values are directly proportional?
Energy and frequency
79
Which energy values are inversely proportional?
Frequency, wavenumber, and wavelength are inversely proportional.
80
How does the 13C NMR chemical shift scale compare to the 'H NMR scale?
The 13C NMR scale is 15-20x larger than the 'H scale, so:
(15-20) \* 'H scale = 13C scale
81
What happens when more than one EWG acts on a given H environment?
Look at each piece, then add the smallest piece + the difference between the smallest and next piece for each piece
82
What is proton decoupled 13C NMR spectroscopy?
NMR with one unsplit peak for each type of carbon environment.
83
What is spin-spin splitting?
Protons within 3 bonds of each other can effect the chemical shift of the neighboring proton environment.
Ex: If Ha has an α-spin, it will slightly increase the chemical shift of Hb. If it has a β-spin, it will slightly cecrease the chemical shift to create a doublet.
84
Relationship between 13C and 'H NMR
γ 13C = 1/4 γ 1H
85
What is a chemical shift?
The distance downfield (left) a peak appears relative to TMS (tertramethyl silane)
86
Given that a NMR has a frequency of 480 Hz, how many Hz downfield is a peak found at 4.3 ppm?
2064
87
Number of Peaks
N + 1
(N = number of hydrogens surrounding proton environment)
88
What does Resonance do to Chemical Shifts?
It distributes the + charge across mutliple carbons, so it causes strong deshielding
89
Remember About O-H and N-H
O-H and N-H hydrogens do not participate in coupling, so do not include in N+1 rule
90
What are coupling constants?
The difference in the chemical shift when protons have α or β spin. It is the gap between the split peaks.
91
Vicinal C-H Coupling Constant
| (free rotation)
Jab = 7 Hz
92
Cis H - C = C - H Coupling Constant
| (vicinal)
Jab = 10 Hz
93
Trans H - C = C - H Coupling Constant
| (vicinal)
Jab = 15 Hz
94
Geminal C = C - H Coupling Constant
Jab = 2 Hz
95
What information is provided by coupling constants?
They help differentiate between cis and trans alkenes
96
What if the coupling constants are similar?
Peaks closer than 0.5 Hz together will appear as one peak with low resolution.
\*\* Need a difference of 0.5 Hz or larger between coupling constants to see splitting of a peak clearly \*\*
97
Diastereotopic
Hydrogen environments are different, making the molecules diastereomers
\*\* Chiral centers indicate distereomers \*\*
98
Enanteotopic
The hydrogen environments are the same, so the molecules are mirror images of each other or enantiomers
99
Homeotopic
The hydrogen environments are the same, so the molecules are the same was well
100
Do we see one or two peaks for axial and equitorial hydrogens on 'H NMR?
At room temperature, we see one peak because it's flipping too quickly to distinguish each chair conformation
At -78°C, we see two peaks because the ring conversion slows enough that the NMR resolution (1/100 sec.) can pick up each chair conformation
\*\* Remember fan blade analogy for explanation \*\*
101
How can ROH and RNH2 peaks be verified?
You can use proton transfers with D2O to see if previous OH and NH peaks will relocate to 4.7
102
Under what circumstances are peaks in a 13C NMR spectrum split?
\*\* Off-Resonance 13C NMR \*\*
The protons directly bonded to the carbon with split the corresponding carbon peak
103
What is DEPT 13C Spectroscopy?
Gives the same information as normal and off-resonance 13C spectroscopy, but in a cleaner fashion. Distinguishes between methine, methylene, and methyl carbons
\*\* DEPT-90 only shows methine (-CH) pointing up \*\*
\*\* DEPT-135 shows methine (-CH) and methyl (-CH3) pointing up, and methylene (-CH2) pointing down \*\*
104
What is COSY NMR?
2D NMR that has 'H NMR on the X and Y axis, which tells you which protons are being split by which protons.
\*\* Used to make more accurate proton assignments within the structure \*\*
105
What is HECTOR NMR?
2D NMR with 13C NMR on the X axis and 'H NMR on the Y axis, that tells you which protons are attached to which carbons
106
MRI
Magnetic resonance imaging
Essentially glorified 'H NMR spectrometer that uses people as the sample, realtively noninvasive because it only uses radio waves
